Federal Income Tax Consequences

        The following is only a brief summary of the material federal income tax aspects of the 401(k) Plan. You should not consider the following as a complete or definitive description of the material federal income tax consequences relating to the 401(k) Plan. Statutory provisions change, as do their interpretations, and their application may vary in individual circumstances. Finally, the consequences under applicable state and local income tax laws may not be the same as under the federal income tax laws. You are urged to consult your tax advisor with respect to any distribution from the 401(k) Plan and transactions involving the 401(k) Plan.

        As a "qualified retirement plan," the Internal Revenue Code affords the 401(k) Plan special tax treatment, including:

(1)

The sponsoring employer is allowed an immediate tax deduction for the amount contributed to the plan each year;



(2)

participants pay no current income tax on amounts contributed by the employer on their behalf; and



(3)

earnings of the plan are tax-deferred, thereby permitting the tax-free accumulation of income and gains on investments.

        HarborOne Bank will administer the 401(k) Plan to comply with the requirements of the Internal Revenue Code. If HarborOne Bank receives an adverse determination letter regarding its tax exempt status from the Internal Revenue Service, all participants would generally recognize income equal to their vested interest in the plan, the participants would not be permitted to transfer amounts distributed from the 401(k) Plan to an individual retirement account or to another qualified retirement plan, and HarborOne Bank may be denied certain deductions taken with respect to the 401(k) Plan.

        Lump Sum Distribution.     A distribution from the 401(k) Plan to a participant or the beneficiary of a participant will qualify as a lump sum distribution if it is made within one taxable year, on account of the participant's death, disability or separation from service, or after the participant attains age 59 ¹ / 2 , and consists of the balance credited to the participant under the plan. The portion of any lump sum distribution required to be included in your taxable income for federal income tax purposes consists of the entire amount of the lump sum distribution.

        HarborOne Bancorp, Inc. Common Stock Included in Lump Sum Distribution.     If a lump sum distribution includes HarborOne Bancorp, Inc. common stock, the distribution generally will be taxed in the manner described above, except that the total taxable amount will be reduced by the amount of any net unrealized appreciation with respect to HarborOne Bancorp, Inc. common stock, that is, the excess of the value of HarborOne Bancorp, Inc. common stock at the time of the distribution over its cost or other basis of the securities to the trust. The tax basis of HarborOne Bancorp, Inc. common stock for computing gain or loss on its subsequent sale equals the value of HarborOne Bancorp, Inc. common stock at the time of distribution, less the amount of net unrealized appreciation. Any gain on a subsequent sale or other taxable disposition of HarborOne Bancorp, Inc. common stock, to the extent of the amount of net unrealized appreciation at the time of distribution, will constitute long-term capital gain regardless of how long the HarborOne Bancorp, Inc. common stock is held. Any gain on a

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subsequent sale or other taxable disposition of HarborOne Bancorp, Inc. common stock in excess of the amount of net unrealized appreciation at the time of distribution will be considered either short-term or long-term capital gain, depending upon the length of the holding period of HarborOne Bancorp, Inc. common stock after the date of distribution from the 401(k) Plan. The recipient of a distribution may elect to include the amount of any net unrealized appreciation in the total taxable amount of the distribution.

        Distributions: Rollovers and Direct Transfers to Another Qualified Plan or to an IRA.     You may roll over virtually all distributions from the 401(k) Plan to another qualified retirement plan or to an individual retirement account.

         We have provided you with a brief description of the material federal income tax aspects of the 401(k) Plan which are of general application under the Internal Revenue Code. It is not intended to be a complete or definitive description of the federal income tax consequences of participating in or receiving distributions from the 401(k) Plan. Accordingly, you are urged to consult a tax advisor concerning the federal, state and local tax consequences of participating in and receiving distributions from the 401(k) Plan.

Additional Employee Retirement Income Security Act Considerations

        As noted above, the 401(k) Plan is subject to certain provisions of ERISA, including special provisions relating to control over the 401(k) Plan's assets by participants and beneficiaries. The 401(k) Plan's feature that allows you to direct the investment of your account balances is intended to satisfy the requirements of section 404(c) of ERISA relating to control over plan assets by a participant or beneficiary. The effect of this is two-fold. First, you will not be deemed a "fiduciary" because of your exercise of investment discretion. Second, no person who otherwise is a fiduciary, such as HarborOne Bank, as the plan administrator, or the 401(k) Plan's trustee is liable under the fiduciary responsibility provision of ERISA for any loss which results from your exercise of control over the assets in your 401(k) Plan account.

        Because you will be entitled to invest a portion of your account balance in the 401(k) Plan in HarborOne Bancorp, Inc. common stock, the regulations under ERISA section 404(c) require that the 401(k) Plan establish procedures that ensure the confidentiality of your decision to purchase, hold, or sell employer securities, except to the extent that disclosure of such information is necessary to comply with federal or state laws not preempted by ERISA. These regulations also require that your exercise of voting and similar rights with respect to the HarborOne Bancorp, Inc. common stock be conducted in a way that ensures the confidentiality of your exercise of these rights.

Restrictions on Resale

        There are restrictions on resale of shares of HarborOne Bancorp, Inc. common stock applicable to persons who may be "affiliates" of HarborOne Bancorp, Inc., including any person receiving a distribution of shares of common stock under the 401(k) Plan who is an "affiliate" of HarborOne Bancorp under the Securities Act of 1933, as amended. An "affiliate" of HarborOne Bancorp, Inc. is someone who directly or indirectly, through one or more intermediaries, controls, is controlled by, or is under common control, with HarborOne Bancorp, Inc. Normally, a director, principal officer or major shareholder of a corporation may be deemed to be an "affiliate" of that corporation. A person who may be deemed an "affiliate" of HarborOne Bancorp, Inc. at the time of a proposed resale will be permitted to make public resales of the common stock only under a "reoffer" prospectus or in accordance with the restrictions and conditions contained in Rule 144 under the Securities Act, or some other exemption from registration, and will not be permitted to use this prospectus in connection with any such resale. In general, the amount of the common stock which any such affiliate may publicly resell under Rule 144 in any three-month period may not exceed the greater of 1% of HarborOne

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Bancorp, Inc. common stock then outstanding or the average weekly trading volume reported on the Nasdaq Global Market during the four calendar weeks before the sale. Such sales may be made only through brokers without solicitation and only at a time when HarborOne Bancorp, Inc. is current in filing the reports required of it under the Securities Exchange Act of 1934, as amended.

         Any person who may be an "affiliate" of HarborOne Bank may wish to consult with counsel before transferring any common stock they own. In addition, participants are advised to consult with counsel as to the applicability of Section 16 of the Securities Exchange Act of 1934, as amended, which may restrict the sale of HarborOne Bancorp, Inc. common stock acquired under the plan, or other sales of HarborOne Bancorp, Inc. common stock.

        Persons who are not deemed to be "affiliates" of HarborOne Bancorp, Inc. at the time of resale will be free to resell any shares of HarborOne Bancorp, Inc. common stock distributed to them under the Plan, either publicly or privately, without regard to the registration and prospectus delivery requirements of the Securities Act, or compliance with the restrictions and conditions contained in the exemptive rules under federal law.

SEC Reporting and Short-Swing Profit Liability

        Section 16 of the Securities Exchange Act, imposes reporting and liability requirements on officers, directors and persons beneficially owning more than 10% of public companies such as HarborOne Bancorp, Inc. Section 16(a) of the Securities Exchange Act, requires the filing of reports of beneficial ownership. Within ten days of becoming a person required to file reports under Section 16(a), a Form 3 reporting initial beneficial ownership must be filed with the Securities and Exchange Commission, or the SEC. Certain changes in beneficial ownership involving allocation or reallocation of assets held in your 401(k) Plan account must be reported periodically, either on a Form 4 within two days after a transaction, or annually on a Form 5 within 45 days after the close of a company's fiscal year.

        In addition to the reporting requirements described above, Section 16(b) of the Securities Exchange Act, or Section 16(b), provides for the recovery by HarborOne Bancorp, Inc. of profits realized by any officer, director or any person beneficially owning more than 10% of the common stock resulting from the purchase and sale or sale and purchase of the common stock within any six-month period.

        The SEC has adopted rules that exempt many transactions involving the Plan from the "short-swing" profit recovery provisions of Section 16(b). The exemptions are available if an election to transfer into the HarborOne Bancorp, Inc. Stock Fund is at least six months after an election to transfer out of the HarborOne Bancorp, Inc. Stock Fund, and vice versa.

        Except for distributions of the common stock due to death, disability, retirement, termination of employment or under a qualified domestic relations order, persons who are governed by Section 16(b) may, under limited circumstances involving the purchase of common stock within six months of the distribution, be required to hold shares of the common stock distributed from the 401(k) Plan for six months following the distribution date.

LEGAL OPINION

        The validity of the issuance of the common stock will be passed upon by Goodwin Procter LLP , Boston, Massachusetts. Goodwin Procter LLP has acted as special counsel for HarborOne Bancorp, Inc. in connection with the reorganization of HarborOne Bank into a two-tier mutual holding company and the related HarborOne Bancorp, Inc. initial public offering.

15

PROSPECTUS SUPPLEMENT

Interests in
Merrimack Mortgage, LLC Retirement Plan
and
Offering of up to 751,162 Shares of

HarborOne Bancorp, Inc.
Common Stock ($0.01 Par Value)

        This prospectus supplement relates to the offer and sale to participants in the Merrimack Mortgage, LLC Retirement Plan, or the 401(k) Plan, of participation interests and shares of common stock of HarborOne Bancorp, Inc.

        401(k) Plan participants may direct the trustee of the 401(k) Plan to use up to 100% of their current account balances to subscribe for and purchase shares of HarborOne Bancorp, Inc. common stock. This prospectus supplement relates to the election of 401(k) Plan participants to direct the trustee of the 401(k) Plan to invest all or a portion of their 401(k) Plan accounts in HarborOne Bancorp, Inc. common stock.

        The prospectus dated [     ·     ] of HarborOne Bancorp, Inc., which accompanies this prospectus supplement, includes detailed information regarding the reorganization of HarborOne Bank into the mutual holding company form of organization, HarborOne Bancorp, Inc.'s stock offering, and the financial condition, results of operations and business of HarborOne Bank. This prospectus supplement provides information regarding the 401(k) Plan. You should read this prospectus supplement together with the accompanying prospectus and keep both for future reference.

         Please refer to "Risk Factors" beginning on page 11 of the accompanying prospectus.

         The securities offered in this prospectus supplement are not deposits or accounts and are not insured or guaranteed by the Federal Deposit Insurance Corporation, any government agency or the Depositors Insurance Fund.

         None of the Securities and Exchange Commission, the Board of Governors of the Federal Reserve System, the Federal Deposit Insurance Corporation, the Massachusetts Commissioner of Banks, nor any other state or federal agency or any state securities regulator, has approved or disapproved of these securities or determined if this prospectus supplement is accurate or complete. Any representation to the contrary is a criminal offense.

        This prospectus supplement may be used only in connection with offers and sales by HarborOne Bancorp, Inc. of interests in, or shares of, common stock acquired by the 401(k) Plan. No one may use this prospectus supplement to reoffer or resell interests in, or shares of, common stock acquired through the 401(k) Plan.

        Neither HarborOne Bancorp, Inc., HarborOne Bank nor Merrimack Mortgage, LLC has authorized any person to give any information or to make any representations other than those contained in the prospectus or this prospectus supplement, and, if given or made, no one may rely on such information or representations as having been authorized by HarborOne Bancorp, Inc., HarborOne Bank or the 401(k) Plan.

        This prospectus supplement does not constitute an offer to sell or solicitation of an offer to buy any securities in any jurisdiction in which it is unlawful to make such offer or solicitation in such jurisdiction. Neither the delivery of this prospectus supplement and the prospectus nor any sale of common stock shall under any circumstances imply that there has been no change in the affairs of HarborOne Bancorp, Inc. or any of its subsidiaries incorporated by the accompanying prospectus or the 401(k) Plan since the date of this prospectus supplement, or imply the information contained in this prospectus supplement is correct as of any time after the date of this prospectus supplement.

The date of this prospectus supplement is [     ·     ].

Table of Contents

TABLE OF CONTENTS

i

THE OFFERING

Securities Offered

        The securities offered in connection with this prospectus supplement are participation interests in the Merrimack Mortgage, LLC Retirement Plan, or the 401(k) Plan. HarborOne Bancorp, Inc., the proposed mid-tier holding company for HarborOne Bank, is the issuer of the common stock. All participants in the 401(k) Plan may use up to 100% of their account balances to subscribe for shares of HarborOne Bancorp, Inc. common stock in the offering subject to the purchase priorities set forth in the HarborOne Bank Plan of Reorganization and Minority Stock Issuance. See "The Reorganization and Offering—General" in the prospectus accompanying this prospectus supplement for a discussion of the purchase priorities in the offering. The interests offered under this prospectus supplement are conditioned on the consummation of the reorganization of HarborOne Bank and related HarborOne Bancorp, Inc. stock offering.

        This prospectus supplement contains information regarding the 401(k) Plan. The accompanying prospectus contains information regarding the reorganization of HarborOne Bank into the mutual holding company form of organization, HarborOne Bancorp, Inc.'s stock offering, and the financial condition, results of operations and business of HarborOne Bank. The address of the principal executive office of HarborOne Bank is 770 Oak Street, Brockton, MA 02301. The telephone number of HarborOne Bank is (508) 895-1000.

Election to Purchase the Common Stock in the Offering; Priorities

        In connection with the offering, the 401(k) Plan provides an additional investment option that allows you to transfer up to 100% of the funds which represent your beneficial interest in the assets of the 401(k) Plan to the HarborOne Bancorp, Inc. Stock Fund. If you elect to invest in the HarborOne Bancorp, Inc. Stock Fund, the 401(k) Plan trustee will subscribe for the common stock offered for sale in the offering in accordance with your direction, subject to the purchase priorities discussed below. In the event the offering is oversubscribed and some or all of your funds cannot be used to purchase common stock in the offering, the 401(k) Plan trustee will return the amount not invested in common stock to the other investment funds in the 401(k) Plan in accordance with your then-existing investment election for future contributions in the 401(k) Plan.

        All plan participants are eligible to direct a transfer of funds to the HarborOne Bancorp, Inc. Stock Fund. However, these directions are subject to subscription rights and purchase priorities. Your order for shares in the offering will be filled based on your purchase priority in the offering. HarborOne Bancorp, Inc. has granted rights to subscribe for shares of HarborOne Bancorp, Inc. common stock to the following persons in the subscription offering, in descending order: (i) depositors who had accounts at HarborOne Bank with aggregate balances of at least $50 as of the close of business on December 31, 2014, (ii) the tax qualified employee benefit plans of HarborOne Bank and its wholly owned subsidiary, Merrimack Mortgage Company, LLC, or "Merrimack Mortgage" (i.e., the HarborOne Employee Stock Ownership Plan, the HarborOne 401(k) Plan and the Merrimack Mortgage, LLC Retirement Plan) and (iii) employees, officers, directors, trustees and corporators of HarborOne Bank or Merrimack Mortgage who do not qualify under priority (i) above. No individual may purchase more than 60,000 shares of common stock in the offering through one or more individual and/or joint deposit accounts, including HarborOne Bancorp, Inc. Stock Fund accounts, and no individual together with any associate or group acting in concert may purchase more than 80,000 shares of common stock.

        If any shares of our common stock remain unsold in the subscription offering, we will offer such shares for sale in a community offering. Natural persons residing in the Massachusetts cities and towns of Abington, Attleboro, Avon, Berkley, Braintree, Bridgewater, Brockton, Canton, Carver, Cohasset, Dighton, Duxbury, East Bridgewater, Easton, Foxboro, Halifax, Hanover, Hanson, Hingham, Holbrook, Hull, Kingston, Lakeville, Mansfield, Marshfield, Middleborough, Milton, North Attleboro, Norton, Norwell, Plainville, Pembroke, Plymouth, Plympton, Quincy, Randolph, Raynham, Rehoboth, Rochester, Rockland, Seekonk, Scituate, Sharon, Stoughton, Taunton, Wareham, West Bridgewater,

Weymouth and Whitman will have a purchase preference in any community offering. Shares also may be offered to the general public. The community offering, if any, may commence concurrently with, during or promptly after, the subscription offering. See "The Reorganization and Offering—Community Offering" in the accompanying prospectus for a discussion of the community offering.

        We also may offer shares of common stock not purchased in the subscription offering or the community offering through a syndicate of brokers in what is referred to as a syndicated community offering. The syndicated community offering, if necessary, would be managed by Sandler O'Neill & Partners, L.P. and would commence as soon as practicable after the termination of the subscription offering and would be open to the general public beyond the local community. We have the right to accept or reject, in our sole discretion, any orders received in the community offering and the syndicated community offering.

        To ensure a proper allocation of stock, each eligible account holder must list on his or her stock purchase order form all deposit accounts in which he or she had an ownership interest at December 31, 2014. Failure to list an account, or providing incorrect information, could result in the loss of all or part of a subscriber's stock allocation. We will strive to identify your ownership in all accounts, but cannot guarantee that we will identify all accounts in which you have an ownership interest. Our interpretation of the terms and conditions of the Plan of Reorganization and of the acceptability of the stock purchase order forms will be final.

        No investor in the offering may purchase fewer than 25 shares of common stock or more than 60,000 shares of common stock.

Value of Participation Interests

        As of December 31, 2015, the market value of the assets of the 401(k) Plan totaled approximately $7.5 million, and up to 100% of this amount may be used to purchase common stock in the offering. The plan administrator informed each participant of the value of his or her beneficial interest in the 401(k) Plan as of December 31, 2015. The value of plan assets represents the past contributions to the 401(k) Plan by or on behalf of the participants of the 401(k) Plan, plus or minus earnings or losses on the contributions, less previous withdrawals.

Method of Directing Transfer

        If you want to use your 401(k) Plan funds to purchase common stock in the offering, you must complete the Special Investment Election Form included with this prospectus supplement and submit the form to Patricia Williams, Senior Vice President in the HarborOne Bank Human Resources Department, by hand delivery, electronic mail, interoffice mail, fax or regular mail, as specified in the first paragraph of the Special Investment Form. The 401(k) Plan trustee will submit an order form on your behalf to purchase the number of shares in the offering that you have elected to purchase. To transfer the funds into the HarborOne Bancorp, Inc. Stock Fund, the 401(k) Plan trustee will withdraw the amount indicated on the Special Investment Election Form from the other investment funds in which your account is invested on a pro rata basis. If you do not wish to purchase shares in the offering with your 401(k) Plan funds, you should still check the applicable box on the Special Investment Election Form included with this prospectus supplement and submit the form to Patricia Williams.

Deadline for Directing Transfer in Connection with the Offering

        The deadline for submitting your instructions to Patricia Williams in the Human Resources Department, to transfer your funds to the HarborOne Bancorp, Inc. Stock Fund in connection with the offering is [    :    ] p.m., Eastern time, on [401(k) Expiration Date].

2

         The 401(k) Plan deadline is earlier than the [Expiration Date] deadline for submitting stock order forms to purchase shares in the offering with funds outside of the 401(k) Plan.

Irrevocability of Transfer Direction in Connection with the Offering

        Your direction to transfer amounts credited to your account in the 401(k) Plan to the HarborOne Bancorp, Inc. Stock Fund in connection with the offering cannot be changed. Pending completion of the offering, the funds you elect to transfer to the HarborOne Bancorp, Inc. Stock Fund will be held in an interest-bearing account until the offering is completed.

Direction to Purchase the Common Stock After the Close of the Offering

        After the close of the offering, you may direct on a daily basis the 401(k) Plan trustee to transfer a certain percentage (in multiples of not less than 1%) of the net value of your interests in the other investment funds into the 401(k) Plan to the HarborOne Bancorp, Inc. Stock Fund. Alternatively, you may direct the 401(k) Plan trustee to transfer a certain percentage of your interest in the HarborOne Bancorp, Inc. Stock Fund into any of the other investment funds in the 401(k) Plan in accordance with the terms of the 401(k) Plan. Following your initial election, you may change the allocation of your interest in the HarborOne Bancorp, Inc. Stock Fund by accessing NetBenefits @ www.netbenefits.com or by calling Fidelity at 1-800-294-4015. Special restrictions may apply to transfers directed by those participants who are officers, directors and 10% shareholders of HarborOne Bancorp, Inc.

Common Stock in the Offering will be Purchased at $10 Per Share

        You will pay the same price for shares of common stock as all other persons who purchase shares of the common stock in the offering. Post-reorganization purchases of common stock in the HarborOne Bancorp, Inc. Stock Fund will be made at prevailing market prices. These prices may be higher or lower than the current offering price of $10 per share of common stock.

Nature of a Participant's Interest in the HarborOne Bancorp, Inc. Stock Fund

        The 401(k) Plan purchases its underlying investments every pay period. Each investment fund's unit value is updated every business day based on the total value of its underlying investments and the number of units held in the fund. Distributions, withdrawals, loans and investment transfers may occur each business day. Loan and transfer requests are made through NetBenefit @ www.netbenefits.com or by calling Fidelity at 1-800-294-4015.

Voting Rights of the Common Stock

        The 401(k) Plan provides that, after the offering, you may direct the 401(k) Plan trustee how to vote the shares of HarborOne Bancorp, Inc. common stock held by the HarborOne Bancorp, Inc. Stock Fund representing the interest in the shares that is credited to your account. If the trustee does not receive your voting instructions, the plan administrator will exercise these rights as it determines in its discretion and will direct the trustee accordingly. All voting instructions will be kept confidential.

3

DESCRIPTION OF THE 401(k) PLAN

Introduction

        Since April 1, 1989, Merrimack Mortgage has maintained the 401(k) Plan. Merrimack Mortgage intends for the 401(k) Plan to comply, in form and in operation, with all applicable provisions of the Internal Revenue Code and the Employee Retirement Income Security Act, most commonly referred to as ERISA. Merrimack Mortgage may amend the 401(k) Plan from time to time to ensure continued compliance with these laws. Merrimack Mortgage may also amend the 401(k) Plan from time to time to add, modify, or eliminate certain features of the plan, as it sees fit. As a plan subject to ERISA, federal law provides you with various rights and protections as a plan participant. Although the 401(k) Plan is subject to many of the provisions of ERISA, your benefits under the plan are not guaranteed by the Pension Benefit Guaranty Corporation.

         Applicable federal tax law requires the 401(k) Plan to impose substantial restrictions on your right to withdraw amounts held under the plan. Generally, withdrawals of 401(k) pre-tax and Roth deferrals are not permitted before a participant's death, disability, attainment of age 59 ¹ / 2 , termination of employment, in the case of certain loans or in connection with a financial hardship. Federal law may also impose an additional 10% tax on withdrawals made from the 401(k) Plan prior to your attainment of age 59 ¹ / 2 , regardless of whether the withdrawal occurs during your employment with HarborOne Bank or after termination of employment.

        Reference to Full Text of Plan.     The following portions of this prospectus supplement summarize certain provisions of the 401(k) Plan. HarborOne Bank qualifies these summaries in their entirety by the full text of the 401(k) Plan. You may obtain copies of the 401(k) Plan document by sending a request to: Patricia Williams, Senior Vice President, Human Resources, HarborOne Bank, 770 Oak Street, Brockton, MA 02301. You should carefully read the full text of the 401(k) Plan document to understand your rights and obligations under the plan.

Eligibility and Participation

        Any eligible employee of Merrimack Mortgage who has completed three months of employment may make pre-tax deferrals into the 401(k) Plan and may be eligible to receive matching employer nonelective contributions. Eligible employees include all employees other than employees covered by a collective bargaining agreement and nonresident aliens with no U.S. source income. Seasonal employees are not eligible to participate unless they attain age 21 and have completed 1,000 hours of service.

        As of December 31, 2015, there were 245 participants in the 401(k) Plan.

Contributions Under the Plan

        401(k) Plan Participant Contributions.     The 401(k) Plan permits each participant to make pre-tax salary deferrals in an amount up to the statutory limit of $18,000 for participants under age 50 and $24,000 for participants age 50 or older. All eligible employees who are not seasonal employees who have completed three months of employment are automatically enrolled in the 401(k) Plan unless they elect otherwise, and 3% of compensation will be withheld from their pay automatically. Participants in the 401(k) Plan may modify the amount contributed to the 401(k) Plan, effective on the first day of the next payroll period, by filing a new deferral agreement with the plan administrator.

        Merrimack Mortgage Contributions.     At the time of initial hire, the plan administrator will provide each new eligible employee with a notice that explains the automatic enrollment provisions. This notice will also explain how an employee may elect not to have automatic deferrals made to the 401(k) Plan or how to alter the amount of the salary deferrals. Merrimack Mortgage may make matching or nonelective contributions to the 401(k) Plan, but it has not done so in the past. If Merrimack Mortgage

4

makes a nonelective contribution, it will be allocated to plan participants who have completed a year of service and who are either employed on the last day of the plan year, or who retired, died or became disabled during the plan year, in the ratio that each eligible participant's compensation bears to the total compensation paid to all eligible participants for the plan year. Only compensation up to $265,000 (as adjusted) is recognized under the 401(k) Plan. If Merrimack Mortgage makes a matching contribution, it will be allocated to participants who make pre-tax salary deferrals.

Limitations on Contributions

        Limitation on Employee Salary Deferral.     Although the 401(k) Plan permits you to defer up to 75% of your compensation, by law, your total pre-tax deferrals under the 401(k) Plan for the 2016 Plan Year, together with contributions to similar plans, may not exceed $18,000, provided, however, if you are over age 50, you may contribute an additional $6,000 per year. The IRS will periodically increase these annual limitations. Contributions in excess of the limitations or excess deferrals, will be included in an affected participant's gross income for federal income tax purposes in the year they are made. In addition, any such excess deferral made on a pre-tax basis will again be subject to federal income tax when distributed by the 401(k) Plan to the participant, unless the excess deferral together with any income allocable thereto, is distributed to the participant not later than the first April 15th following the close of the taxable year in which the excess deferral is made. Any income on the excess deferral that is distributed not later than such date shall be treated, for federal income tax purposes, as earned and received by the participant in the taxable year in which the distribution is made.

        Limitations on Annual Additions and Benefits.     Under the requirements of the Internal Revenue Code, the 401(k) Plan provides that the total amount of contributions and forfeitures (annual additions) allocated to participants under the 401(k) Plan and other defined contribution plans maintained by Merrimack Mortgage during any plan year may not exceed the lesser of 100% of the participant's compensation, or $53,000 (as indexed for future years for increases in the cost of living).

        Top-Heavy Plan Requirements.     If for any calendar year the 401(k) Plan is a Top-Heavy Plan, then HarborOne Bank may be required to make certain minimum contributions to the Plan on behalf of non-key employees.

        In general, the 401(k) Plan will be treated as a "Top-Heavy Plan" for any calendar year if, as of the last day of the preceding calendar year, the aggregate balance of the accounts of participants who are "Key Employees" exceeds 60% of the aggregate balance of the accounts of all participants. Key Employees generally include any employee who, at any time during the calendar year or any of the four preceding years, is:

(1)

an officer of Merrimack Mortgage having annual compensation in excess of $170,000,



(2)

a person who owns, directly or indirectly, more than 5% of the stock of HarborOne Bancorp, Inc., or stock possessing more than 5% of the total combined voting power of all stock of HarborOne Bancorp, Inc., or



(3)

a person who owns, directly or indirectly, combined voting power of all stock and more than 1% of the total stock of HarborOne Bancorp, Inc. and has annual compensation from Merrimack Mortgage in excess of $150,000.

The foregoing dollar amounts are for 2016 and may be adjusted periodically by the IRS.

Investment of Contributions

        All amounts credited to participants' accounts under the 401(k) Plan are held in trust. A trustee appointed by the Board of Managers of Merrimack Mortgage administers the trust.

5

        Prior to the effective date of the offering, you were provided the opportunity to direct the investment of your account into one or more of the following options:

•

Fidelity Money Market Trust Retirement Government Money Market II



•

Fidelity Spartan International Index Fund Investor Class



•

Fidelity Spartan 500 Index Advantage



•

Fidelity Spartan Mid Cap Index Advantage



•

Fidelity Spartan Small Cap Index Advantage



•

Victory Sycamore Established Value A



•

PIMCO Total Return Fund Class A



•

PIMCO Real Return Administrative Class



•

Invesco Comstock Fund Class A



•

Invesco Equity and Income Fund Class A



•

JPMorgan Small Cap Value Fund Class A



•

JPMorgan Large Cap Growth Fund Class A



•

Oppenheimer Global Fund Class A



•

ClearBridge Small Cap Growth Fund Class A



•

AB Discovery Growth Fund Class A



•

Franklin Rising Dividends Fund Class A



•

Franklin Strategic Income Fund Class A



•

T. Rowe Price Retirement 2005 Fund Class R



•

T. Rowe Price Retirement 2010 Fund Class R



•

T. Rowe Price Retirement 2015 Fund Class R



•

T. Rowe Price Retirement 2020 Fund Class R



•

T. Rowe Price Retirement 2025 Fund Class R



•

T. Rowe Price Retirement 2030 Fund Class R



•

T. Rowe Price Retirement 2035 Fund Class R



•

T. Rowe Price Retirement 2040 Fund Class R



•

T. Rowe Price Retirement 2045 Fund Class R



•

T. Rowe Price Retirement 2050 Fund Class R



•

T. Rowe Price Retirement 2055 Fund Class R



•

T. Rowe Price Retirement Balanced Fund Class R

        HarborOne Bancorp, Inc. Stock Fund.     In connection with the offering, the 401(k) Plan now offers the HarborOne Bancorp,  Inc. Stock Fund as an additional investment alternative. The HarborOne Bancorp, Inc. Stock Fund will invest primarily in the common stock of HarborOne Bancorp, Inc. This fund will maintain a relatively small position in a money market account to accommodate liquidity. In connection with the offering, participants in the 401(k) Plan may direct the trustee to invest up to

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100% of their 401(k) Plan account balance in the HarborOne Bancorp, Inc. Stock Fund. Your interest in the HarborOne Bancorp, Inc. Stock Fund will be credited to your 401(k) Plan account in units, just like the other funds available under the 401(k) Plan.

        Subsequent to the offering you may elect (in increments of 1%) to have both past and future contributions and additions to your accounts invested in the HarborOne Bancorp, Inc. Stock Fund and you may also elect to transfer into the HarborOne Bancorp, Inc. Stock Fund a portion of your accounts currently invested in other funds under the 401(k) Plan.

        After the offering, the trustee of the 401(k) Plan will, to the extent practicable, use all amounts held by it in the HarborOne Bancorp, Inc. Stock Fund, including cash dividends paid on the common stock held in the fund, to purchase additional shares of common stock of HarborOne Bancorp, Inc.

        As of the date of this prospectus supplement, none of the shares of common stock have been issued or are outstanding and there is no established market for the common stock of HarborOne Bancorp, Inc. Accordingly, there is no record of the historical performance of the HarborOne Bancorp, Inc. Stock Fund. Performance of the HarborOne Bancorp, Inc. Stock Fund depends on a number of factors, including the financial condition and profitability of HarborOne Bancorp, Inc. and HarborOne Bank and market conditions for the common stock generally.

         Investments in the HarborOne Bancorp, Inc. Stock Fund may involve certain special risks in investments in the common stock of HarborOne Bancorp, Inc. For a discussion of these risk factors, see "Risk Factors" in the accompanying prospectus.

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Performance History

        The following table provides performance data with respect to the investment accounts available under the 401(k) Plan through December 31, 2015 (performance results shown are net of investment management fees):

        The following is a description of each of the Plan's investment options (excerpted from each option's own description):

Fidelity Money Market Trust Retirement Government Money Market II —This fund seeks to obtain as high a level of current income as is consistent with the preservation of capital and liquidity. The Adviser normally invests at least 99.5% of the fund's total assets in cash, U.S. government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Certain issuers of U.S. government securities are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury.

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Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, and diversification of investments.

Fidelity Spartan International Index Fund Investor Class —This fund to provide investment results that correspond to the total return of foreign stock markets. The fund normally invests at least 80% of assets in common stocks included in the Morgan Stanley Capital International Europe, Australasia, Far East Index, which represents the performance of foreign stock markets.

Fidelity Spartan 500 Index Advantage —This fund seeks to provide investment results that correspond to the total return (i.e., the combination of capital changes and income) performance of common stocks publicly traded in the United States. The fund normally invests at least 80% of assets in common stocks included in the S&P 500 Index, which broadly represents the performance of common stocks publicly traded in the United States.

Fidelity Spartan Mid Cap Index Advantage —this fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization United States companies. The fund normally invests at least 80% of assets in securities included in the Russell Midcap Index as well as lends securities to earn income for the fund.

Fidelity Spartan Small Cap Index Advantage —This fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization U.S. companies. The fund normally invests at least 80% of assets in securities included in the Russell 2000 Index as well as lends securities to earn income for the fund.

Victory Sycamore Established Value A —This fund seeks to provide long-term capital growth by investing primarily in common stocks. The fund pursues its investment objective by investing, under normal circumstances, at least 80% of its net assets in equity securities of companies with market capitalizations, at the time of purchase, within the range of companies comprising the Russell MidCap Value Index. It may invest a portion of its assets in equity securities of foreign companies traded on U.S. exchanges, including American and Global Depositary Receipts (ADRs and GDRs).

PIMCO Total Return Fund Class A —This fund seeks maximum total return, consistent with prudent investment management. The fund seeks to achieve its investment objective by investing under normal circumstances at least 65% of its total assets in a diversified portfolio of Fixed Income Instruments of varying maturities, which may be represented by forwards or derivatives such as options, futures contracts or swap agreements. It invests primarily in investment grade debt securities, but may invest up to 10% of its total assets in high yield securities ("junk bonds") that are rated B or higher. The fund may invest up to 30% of its total assets in securities denominated in foreign currencies, and may invest beyond this limit in U.S. dollar-denominated securities of foreign issuers.

PIMCO Real Return Administrative Class —This fund seeks maximum real return, consistent with preservation of capital and prudent investment management. The fund seeks its investment objective by investing under normal circumstances at least 80% of its net assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and corporations, which may be represented by forwards or derivatives such as options, futures contracts or swap agreements. It invests primarily in investment grade securities, but may invest up to 10% of its total assets in high yield securities ("junk bonds") rated B or higher. The fund also may invest up to 30% of its total assets in securities denominated in foreign currencies, and may invest beyond this limit in U.S. dollar denominated securities of foreign issuers.

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Invesco Comstock Fund Class A —This fund seeks total return through growth of capital and current income. The fund invests, under normal circumstances, at least 80% of its net assets (plus any borrowings for investment purposes) in common stocks, and in derivatives and other instruments that have economic characteristics similar to such securities. It may invest in securities of issuers of any market capitalization; however, a substantial number of the issuers in which the fund invests are large-capitalization issuers. The fund may invest up to 10% of its net assets in real estate investment trusts (REITs).

Invesco Equity and Income Fund Class A —This fund seeks to provide current income and, secondarily, capital appreciation. The fund invests, under normal circumstances, at least 80% of its net assets (plus any borrowings for investment purposes) in equity and income securities, and in derivatives and other instruments that have economic characteristics similar to such securities. It invests, under normal circumstances, at least 65% of its net assets in income-producing equity investments, such as dividend paying common or preferred stocks. The fund may invest up to 25% of its net assets in securities of foreign issuers or depositary receipts.

JPMorgan Small Cap Value Fund Class A —This fund seeks long-term capital growth primarily by investing in equity securities of small-capitalization companies. Under normal circumstances, the fund invests at least 80% of its assets in equity securities of small cap companies. Small cap companies are companies with market capitalizations equal to those within the universe of the Russell 2000 Value Index stocks at the time of purchase. In reviewing investment opportunities for the fund, its adviser uses a value-oriented approach. In implementing its main strategies, the fund's equity investments are primarily in common stocks and real estate investment trusts (REITs).

JPMorgan Large Cap Growth Fund Class A —This fund seeks long-term capital appreciation. Under normal circumstances, at least 80% of the fund's assets will be invested in the equity securities of large, well-established companies. "Assets" means net assets, plus the amount of borrowings for investment purposes. Large, well-established companies are companies with market capitalizations equal to those within the universe of the Russell 1000 Growth Index at the time of purchase. Typically, in implementing its strategy, the fund invests in common stocks of companies with a history of above-average growth or companies expected to enter periods of above-average growth.

Oppenheimer Global Fund Class A —This fund seeks capital appreciation. The fund invests mainly in common stock of U.S. and foreign companies. It can invest without limit in foreign securities and can invest in any country, including countries with developing or emerging markets. However, the fund currently emphasizes its investments in developed markets such as the United States, Western European countries and Japan. It does not limit its investments to companies in a particular capitalization range, but primarily invests in mid- and large-cap companies. The fund normally will invest in at least three countries (one of which may be the United States).

ClearBridge Small Cap Growth Fund Class A —This fund seeks long-term growth of capital. Under normal circumstances, the fund invests at least 80% of its assets in equity securities of companies with small market capitalizations and related investments. The portfolio managers use a growth-oriented investment style that emphasizes small U.S. companies.

AB Discovery Growth Fund Class A —This fund seeks to provide long-term growth of capital. The fund invests primarily in a diversified portfolio of equity securities with relatively smaller capitalizations as compared to the overall U.S. market. Under normal circumstances, the fund invests at least 80% of its net assets in the equity securities of small- and mid-capitalization companies. For these purposes, "small- and mid-capitalization companies" are generally those companies that, at the time of investment, fall within the lowest 25% of the total U.S. equity market capitalization (excluding, for purposes of this calculation, companies with market capitalizations of less than $10 million).

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Franklin Rising Dividends Fund Class A —This fund seeks to provide long-term capital appreciation, and preservation of capital, while not a goal, is also an important consideration. Under normal market conditions, the fund invests at least 80% of its net assets in investments of companies that have paid consistently rising dividends. It invests predominantly in equity securities, primarily common stock. The fund may invest in companies of any size, across the entire market spectrum. It may invest up to 25% of its total assets in foreign securities.

Franklin Strategic Income Fund Class A —This fund seeks to earn a high level of current income. A secondary goal is capital appreciation over the long term. Under normal market conditions, the fund invests at least 65% of its assets in U.S. and foreign debt securities, including those in emerging markets. Debt securities include all varieties of fixed and floating rate income securities, including bonds, U.S. and foreign government and agency securities, corporate loans (and loan participations), mortgage-backed securities and other asset-backed securities, convertible securities, and municipal securities.

T. Rowe Price Retirement 2005 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2005) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2010 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2010) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2015 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2015) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2020 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2020) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2025 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond

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funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2025) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2030 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2030) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2035 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2035) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2040 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2040) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2045 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2045) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2050 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2050) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

T. Rowe Price Retirement 2055 Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund pursues its objective by investing in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund's allocation between T. Rowe Price stock and bond funds will change over time in relation to its target retirement date. It is managed based on the specific retirement year (target date 2055) included in its name and assumes a retirement age of 65. While the fund is non-diversified, it invests in diversified underlying holdings.

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T. Rowe Price Retirement Balanced Fund Class R —This fund seeks the highest total return over time consistent with an emphasis on both capital growth and income. The fund invests in a diversified portfolio of other T. Rowe Price stock and bond funds that represent various asset classes and sectors. The fund is intended for retired investors who seek income and relative stability from bonds along with some capital appreciation potential from stocks. The fund's "neutral allocations," which are what T. Rowe Price considers broadly appropriate for investors during their retirement years, are 40% stock funds and 60% bond funds. While the fund is non-diversified, it invests in diversified underlying holdings.

         An investment in any of the funds listed above is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. As with any mutual fund investment, there is always a risk that you may lose money on your investment in any of the funds listed above.

Benefits Under the Plan

        Vesting.     At all times, you have a fully vested, nonforfeitable interest in your own contributions, adjusted for earnings or losses. Your share of employer nonelective contributions, if any, as adjusted for earnings or losses, becomes vested at 25% after one year of service, and vesting increases by 25% for each subsequent year of service until you become fully vested after four years of service.

Withdrawals and Distributions From the 401(k) Plan

        Withdrawals Prior to Termination of Employment.     You may receive in-service distributions from the 401(k) Plan. The 401(k) Plan permits in-service withdrawals of rollover contributions at any time. You may also apply for a hardship withdrawal from your salary deferral contributions and employer contributions if you are under age 59 ¹ / 2 . In order to qualify for a hardship withdrawal, you must have an immediate and substantial need to meet certain expenses and have no other reasonably available resources to meet the financial need. If you qualify for a hardship distribution, the trustee will make the distribution pro rata from the investment funds in which you have invested your own contributions. If you make a hardship withdrawal, your right to make elective deferrals is suspended for six months. Hardship withdrawals may not be paid back to the 401(k) Plan. Once you attain age 59 ¹ / 2 , all your vested account balances in the 401(k) Plan are eligible for in-service withdrawals. You can also apply for a loan from the 401(k) Plan. You cannot have more than one loan outstanding at a time. You can apply for a minimum loan of $1,000 and a maximum loan of the lesser of $50,000 or 50% of your total vested account balance.

        Distribution Upon Retirement or Disability.     Participants shall receive benefits as soon as administratively feasible following the close of a valuation period during which the distribution is requested. Distributions are payable to participants in a lump sum or installments, at the participant's election. Certain distributions may also be rolled over to another qualified plan or individual retirement account.

        Distribution Upon Death.     If you die prior to your benefits being paid from the 401(k) Plan, your benefits will be paid to your surviving spouse or beneficiary under one or more of the forms available under the 401(k) Plan. Certain distributions may also be rolled over to another qualified plan or individual retirement account.

        Distribution Upon Termination for Any Other Reason.     If you terminate employment for any reason other than retirement, disability or death and your vested account balance exceeds $5,000, the trustee will make your distribution when you attain age 65, unless you request otherwise. If your vested account balance does not exceed $1,000, the trustee will generally distribute your benefits to you as soon as administratively practicable following your termination of employment. If your vested account

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balance is over $1,000 but not more than $5,000, and you have not attained age 65, if you do not make a timely distribution or direct rollover election, your entire vested account balance will automatically be rolled over to a traditional individual retirement account.

        Nonalienation of Benefits.     Except with respect to federal income tax withholding, federal tax liens and as provided with respect to a qualified domestic relations order, benefits payable under the 401(k) Plan shall not be subject in any manner to anticipation, alienation, sale, transfer, assignment, pledge, encumbrance, charge, garnishment, execution, or levy of any kind, either voluntary or involuntary, and any attempt to anticipate, alienate, sell, transfer, assign, pledge, encumber, charge or otherwise dispose of any rights to benefits payable under the 401(k) Plan shall be void.

         Applicable federal tax law requires the 401(k) Plan to impose substantial restrictions on your right to withdraw amounts held under the plan before your termination of employment with Merrimack Mortgage or its affiliates. Federal law may also impose an additional 10% tax on distributions from the 401(k) Plan before you attain 59 ¹ / 2 years of age, regardless of whether the withdrawal occurs during your employment with Merrimack Mortgage or its affiliates or after your termination of employment.

Administration of the Plan

        Trustee.     The board of managers of Merrimack Mortgage has appointed Fidelity Management Trust Company as trustee of the 401(k) Plan. The trustee receives, holds and invests the contributions to the 401(k) Plan in trust and distributes them to participants and beneficiaries in accordance with the terms of the plan and the directions of the plan administrator. The trustee is responsible for investment of the assets of the trust as directed by plan participants.

Reports to Plan Participants

        The plan administrator will furnish you a statement at least quarterly showing (i) the balance in your account as of the end of that period, (ii) the amount of contributions allocated to your account for that period, and (iii) the adjustments to your account to reflect earnings or losses (if any).

Plan Administrator

        Currently, the plan administrator of the 401(k) Plan is Merrimack Mortgage. The plan administrator is responsible for the administration of the 401(k) Plan, interpretation of the provisions of the plan, prescribing procedures for filing applications for benefits, preparation and distribution of information explaining the plan, maintenance of plan records, books of account and all other data necessary for the proper administration of the plan, and preparation and filing of all returns and reports relating to the plan which are required to be filed with the U.S. Department of Labor and the Internal Revenue Service, and for all disclosures required to be made to participants, beneficiaries and others under ERISA.

Amendment and Termination

        Merrimack Mortgage intends to continue the 401(k) Plan indefinitely. Nevertheless, Merrimack Mortgage may terminate the 401(k) Plan at any time. If Merrimack Mortgage terminates the 401(k) Plan in whole or in part, then regardless of other provisions in the plan, all participants affected by such termination shall become fully vested in their accounts. Merrimack Mortgage reserves the right to make, from time to time, any amendment or amendments to the 401(k) Plan which do not cause any part of the trust to be used for, or diverted to, any purpose other than the exclusive benefit of participants or their beneficiaries; provided, however, that Merrimack Mortgage may amend the plan as it determines necessary or desirable, with or without retroactive effect, to comply with ERISA or the Internal Revenue Code.

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Merger, Consolidation or Transfer

        In the event of the merger or consolidation of the 401(k) Plan with another plan, or the transfer of the trust assets to another plan, the plan requires that you would (if either the plan or the other plan then terminated) receive a benefit immediately after the merger, consolidation or transfer which is equal to or greater than the benefit you would have been entitled to receive immediately before the merger, consolidation or transfer (if the plan had then terminated).

Federal Income Tax Consequences

        The following is only a brief summary of the material federal income tax aspects of the 401(k) Plan. You should not consider the following as a complete or definitive description of the material federal income tax consequences relating to the 401(k) Plan. Statutory provisions change, as do their interpretations, and their application may vary in individual circumstances. Finally, the consequences under applicable state and local income tax laws may not be the same as under the federal income tax laws. You are urged to consult your tax advisor with respect to any distribution from the 401(k) Plan and transactions involving the 401(k) Plan.

        As a "qualified retirement plan," the Internal Revenue Code affords the 401(k) Plan special tax treatment, including:

(1)

The sponsoring employer is allowed an immediate tax deduction for the amount contributed to the plan each year;



(2)

participants pay no current income tax on amounts contributed by the employer on their behalf; and



(3)

earnings of the plan are tax-deferred, thereby permitting the tax-free accumulation of income and gains on investments.

        Merrimack Mortgage will administer the 401(k) Plan to comply with the requirements of the Internal Revenue Code. If Merrimack Mortgage receives an adverse determination letter regarding its tax exempt status from the Internal Revenue Service, all participants would generally recognize income equal to their vested interest in the plan, the participants would not be permitted to transfer amounts distributed from the 401(k) Plan to an individual retirement account or to another qualified retirement plan, and Merrimack Mortgage may be denied certain deductions taken with respect to the 401(k) Plan.

        Lump Sum Distribution.     A distribution from the 401(k) Plan to a participant or the beneficiary of a participant will qualify as a lump sum distribution if it is made within one taxable year, on account of the participant's death, disability or separation from service, or after the participant attains age 59 ¹ / 2 , and consists of the balance credited to the participant under the plan. The portion of any lump sum distribution required to be included in your taxable income for federal income tax purposes consists of the entire amount of the lump sum distribution.

        HarborOne Bancorp, Inc. Common Stock Included in Lump Sum Distribution.     If a lump sum distribution includes HarborOne Bancorp, Inc. common stock, the distribution generally will be taxed in the manner described above, except that the total taxable amount will be reduced by the amount of any net unrealized appreciation with respect to HarborOne Bancorp, Inc. common stock, that is, the excess of the value of HarborOne Bancorp, Inc. common stock at the time of the distribution over its cost or other basis of the securities to the trust. The tax basis of HarborOne Bancorp, Inc. common stock for computing gain or loss on its subsequent sale equals the value of HarborOne Bancorp, Inc. common stock at the time of distribution, less the amount of net unrealized appreciation. Any gain on a subsequent sale or other taxable disposition of HarborOne Bancorp, Inc. common stock, to the extent of the amount of net unrealized appreciation at the time of distribution, will constitute long-term

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capital gain regardless of how long the HarborOne Bancorp, Inc. common stock is held. Any gain on a subsequent sale or other taxable disposition of HarborOne Bancorp, Inc. common stock in excess of the amount of net unrealized appreciation at the time of distribution will be considered either short-term or long-term capital gain, depending upon the length of the holding period of HarborOne Bancorp, Inc. common stock after the date of distribution from the 401(k) Plan. The recipient of a distribution may elect to include the amount of any net unrealized appreciation in the total taxable amount of the distribution.

        Distributions: Rollovers and Direct Transfers to Another Qualified Plan or to an IRA.     You may roll over virtually all distributions from the 401(k) Plan to another qualified retirement plan or to an individual retirement account.

         We have provided you with a brief description of the material federal income tax aspects of the 401(k) Plan which are of general application under the Internal Revenue Code. It is not intended to be a complete or definitive description of the federal income tax consequences of participating in or receiving distributions from the 401(k) Plan. Accordingly, you are urged to consult a tax advisor concerning the federal, state and local tax consequences of participating in and receiving distributions from the 401(k) Plan.

Additional Employee Retirement Income Security Act Considerations

        As noted above, the 401(k) Plan is subject to certain provisions of ERISA, including special provisions relating to control over the 401(k) Plan's assets by participants and beneficiaries. The 401(k) Plan's feature that allows you to direct the investment of your account balances is intended to satisfy the requirements of section 404(c) of ERISA relating to control over plan assets by a participant or beneficiary. The effect of this is two-fold. First, you will not be deemed a "fiduciary" because of your exercise of investment discretion. Second, no person who otherwise is a fiduciary, such as HarborOne Bank, as the plan administrator, or the 401(k) Plan's trustee is liable under the fiduciary responsibility provision of ERISA for any loss which results from your exercise of control over the assets in your 401(k) Plan account.

        Because you will be entitled to invest a portion of your account balance in the 401(k) Plan in HarborOne Bancorp, Inc. common stock, the regulations under ERISA section 404(c) require that the 401(k) Plan establish procedures that ensure the confidentiality of your decision to purchase, hold, or sell employer securities, except to the extent that disclosure of such information is necessary to comply with federal or state laws not preempted by ERISA. These regulations also require that your exercise of voting and similar rights with respect to the HarborOne Bancorp, Inc. common stock be conducted in a way that ensures the confidentiality of your exercise of these rights.

Restrictions on Resale

        There are restrictions on resale of shares of HarborOne Bancorp, Inc. common stock applicable to persons who may be "affiliates" of HarborOne Bancorp, Inc., including any person receiving a distribution of shares of common stock under the 401(k) Plan who is an "affiliate" of HarborOne Bancorp under the Securities Act of 1933, as amended. An "affiliate" of HarborOne Bancorp, Inc. is someone who directly or indirectly, through one or more intermediaries, controls, is controlled by, or is under common control, with HarborOne Bancorp, Inc. Normally, a director, principal officer or major shareholder of a corporation may be deemed to be an "affiliate" of that corporation. A person who may be deemed an "affiliate" of HarborOne Bancorp, Inc. at the time of a proposed resale will be permitted to make public resales of the common stock only under a "reoffer" prospectus or in accordance with the restrictions and conditions contained in Rule 144 under the Securities Act, or some other exemption from registration, and will not be permitted to use this prospectus in connection with any such resale. In general, the amount of the common stock which any such affiliate may publicly

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resell under Rule 144 in any three-month period may not exceed the greater of 1% of HarborOne Bancorp, Inc. common stock then outstanding or the average weekly trading volume reported on the Nasdaq Global Market during the four calendar weeks before the sale. Such sales may be made only through brokers without solicitation and only at a time when HarborOne Bancorp, Inc. is current in filing the reports required of it under the Securities Exchange Act of 1934, as amended.

         Any person who may be an "affiliate" of HarborOne Bank may wish to consult with counsel before transferring any common stock they own. In addition, participants are advised to consult with counsel as to the applicability of Section 16 of the Securities Exchange Act of 1934, as amended, which may restrict the sale of HarborOne Bancorp, Inc. common stock acquired under the plan, or other sales of HarborOne Bancorp, Inc. common stock.

        Persons who are not deemed to be "affiliates" of HarborOne Bancorp, Inc. at the time of resale will be free to resell any shares of HarborOne Bancorp, Inc. common stock distributed to them under the Plan, either publicly or privately, without regard to the registration and prospectus delivery requirements of the Securities Act, or compliance with the restrictions and conditions contained in the exemptive rules under federal law.

SEC Reporting and Short-Swing Profit Liability

        Section 16 of the Securities Exchange Act, imposes reporting and liability requirements on officers, directors and persons beneficially owning more than 10% of public companies such as HarborOne Bancorp, Inc. Section 16(a) of the Securities Exchange Act, requires the filing of reports of beneficial ownership. Within ten days of becoming a person required to file reports under Section 16(a), a Form 3 reporting initial beneficial ownership must be filed with the Securities and Exchange Commission, or the SEC. Certain changes in beneficial ownership involving allocation or reallocation of assets held in your 401(k) Plan account must be reported periodically, either on a Form 4 within two days after a transaction, or annually on a Form 5 within 45 days after the close of a company's fiscal year.

        In addition to the reporting requirements described above, Section 16(b) of the Securities Exchange Act, or Section 16(b), provides for the recovery by HarborOne Bancorp, Inc. of profits realized by any officer, director or any person beneficially owning more than 10% of the common stock resulting from the purchase and sale or sale and purchase of the common stock within any six-month period.

        The SEC has adopted rules that exempt many transactions involving the Plan from the "short-swing" profit recovery provisions of Section 16(b). The exemptions are available if an election to transfer into the HarborOne Bancorp, Inc. Stock Fund is at least six months after an election to transfer out of the HarborOne Bancorp, Inc. Stock Fund, and vice versa.

        Except for distributions of the common stock due to death, disability, retirement, termination of employment or under a qualified domestic relations order, persons who are governed by Section 16(b) may, under limited circumstances involving the purchase of common stock within six months of the distribution, be required to hold shares of the common stock distributed from the 401(k) Plan for six months following the distribution date.

LEGAL OPINION

        The validity of the issuance of the common stock will be passed upon by Goodwin Procter LLP , Boston, Massachusetts. Goodwin Procter LLP has acted as special counsel for HarborOne Bancorp, Inc. in connection with the reorganization of HarborOne Bank into a two-tier mutual holding company and the related HarborOne Bancorp, Inc. initial public offering.

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Table of Contents

SUBSCRIPTION AND COMMUNITY
OFFERING PROSPECTUS

HarborOne Bancorp, Inc.

(Proposed Holding Company for HarborOne Bank)
Up to 11,783,475 Shares of Common Stock
(Subject to Increase to up to 13,550,996 Shares)

          HarborOne Bancorp, Inc., a newly formed Massachusetts corporation, is offering common stock for sale in connection with the reorganization of HarborOne Bank into the mutual holding company form of organization. The shares we are offering for sale will represent 45.0% of our outstanding shares of common stock. We also intend to contribute cash equal to 0.667% of the gross proceeds raised in the offering and 1.2% of our outstanding shares of common stock to a new charitable foundation to be established in connection with the reorganization. The remainder of our outstanding shares of common stock will be owned by a newly formed mutual holding company that will be our majority shareholder. We expect that our common stock will be listed on the Nasdaq Global Market under the symbol "HONE." There is currently no public market for the shares of our common stock. We are an "emerging growth company" as defined in the Jumpstart Our Business Startups Act of 2012 and, as such, we have elected to comply with certain reduced public company reporting requirements for this prospectus and future filings.

          We are offering up to 11,783,475 shares of common stock for sale. We must sell a minimum of 8,709,525 shares in order to complete the offering. We may sell up to 13,550,996 shares of common stock because of demand for the shares or changes in market conditions without resoliciting subscribers.

          We are offering the shares of common stock in a subscription offering to eligible depositors of HarborOne Bank. Shares of common stock not purchased in the subscription offering may be offered for sale to the general public in a community offering, with a preference given to residents of the communities served by HarborOne Bank. Any shares of common stock not purchased in the subscription or community offerings may be offered to the general public in a syndicated community offering to be managed by Sandler O'Neill & Partners, L.P. Sandler O'Neill will assist us in selling the shares in the subscription offering and any community or syndicated community offering on a best efforts basis, but is not required to purchase any shares of the common stock that are being offered for sale in those offerings. Any shares not purchased in the subscription, community and syndicated community offerings may be offered for sale in a separate firm commitment offering that will be managed by Sandler O'Neill.

          The minimum order is 25 shares. Stock orders must be received by us before [    :    ] p.m., Eastern time, on [Expiration Date]. We may extend this expiration date without notice to you until [Extension Date #1], unless we receive regulatory approval to extend the offering to a later date, which may not be beyond [Extension Date #2]. Once submitted, orders are irrevocable unless the offering is terminated or is extended beyond [Extension Date #1], or the number of shares of common stock to be sold is increased to more than 13,550,996 shares or decreased to fewer than 8,709,525 shares. If the offering is extended past [Extension Date #1], or the number of shares of common stock to be sold is increased to more than 13,550,996 shares or decreased to fewer than 8,709,525 shares, we will resolicit subscribers, giving them an opportunity to change or cancel their orders. Funds received during the offering will be held in a segregated account at HarborOne Bank, and will earn interest at our statement savings rate, which is currently 0.07% per annum.

          We expect that our directors and executive officers, together with their associates, will subscribe for approximately 332,500 shares of our common stock.

           This investment involves a high degree of risk, including the possible loss of your investment. Please read the section of this prospectus entitled "Risk Factors" beginning on page 11.