PROSPECTUS SUMMARY

         This summary highlights selected information from this proxy statement/prospectus/information statement and may not contain all of the information that is important to you. To better understand the Merger, the proposals being considered at the Special Meeting and Millendo Stockholder actions that are the subject of the written consent, you should read this entire proxy statement/prospectus/information statement carefully, including the Merger Agreement, as amended, attached as Annex A, the opinions of Ladenburg Thalmann & Co. Inc. ("Ladenburg Thalmann") attached as Annex B-1 and B-2, and the other annexes to which you are referred herein. For more information, please see the section titled "Where You Can Find More Information" in this proxy statement/prospectus/information statement.

The Companies

OvaScience, Inc.

OvaScience, Inc.
9 Fourth Avenue
Waltham, MA 02451
(617) 500-2802

        OvaScience is a company focused on the discovery and development of new treatment options for women and families struggling with infertility. To date OvaScience has been leveraging the breakthrough discovery of egg precursor, or EggPC ^SM , cells to transform the treatment landscape for women's fertility. OvaScience's operations to date have been limited to organizing and staffing the company, business planning, raising capital, acquiring and developing our technology, identifying potential fertility treatments, developing the OvaPrime ^SM treatment, the OvaTure ^SM treatment and the AUGMENT ^SM treatment, introducing AUGMENT in select international in vitro fertilization ("IVF") clinics and determining the regulatory and development path for OvaScience's fertility treatments. OvaScience has generated limited revenues to date, and does not anticipate significant revenues in the near term. In June 2017, OvaScience announced that it would continue to focus on advancing OvaPrime in clinical development and OvaTure in preclinical development and would discontinue ongoing efforts related to the AUGMENT treatment outside of North America. To better align its organization with these strategic priorities, OvaScience restructured its workforce and reduced its workforce by approximately 50%. On January 3, 2018, OvaScience announced a further restructuring of its organization and a workforce reduction of approximately 50%. On May 3, 2018, OvaScience announced that its Board of Directors had approved a corporate restructuring plan furthering its on-going efforts to effectively align its resources. On August 9, 2018, OvaScience announced that it had entered into a definitive agreement with Millendo under which Millendo will merge with OvaScience in an all-stock transaction.

Millendo Therapeutics, Inc.

Millendo Therapeutics, Inc.
301 N. Main Street
Suite 100
Ann Arbor, MI 48104
(734) 845-9000

        Millendo is a late-stage biopharmaceutical company focused on developing novel treatments for orphan endocrine diseases where current therapies do not exist or are insufficient. The endocrine system is a collection of glands that secrete hormones into the blood stream to regulate a number of functions, including appetite, metabolism, growth, development and reproduction. Diseases of the endocrine system can cause multiple and varied symptoms, including appetite dysregulation, metabolic

dysfunction, obesity, cardiovascular disease, menstrual irregularity, excessive facial and body hair growth, and infertility.

        Millendo is currently advancing two product candidates to treat three indications. Millendo's most advanced product candidate, livoletide (AZP-531), is a potential treatment for Prader-Willi syndrome, or PWS, a rare and complex genetic endocrine disease characterized by hyperphagia, or insatiable hunger, that contributes to serious complications, a significant burden on patients and caregivers and early mortality. In a randomized, double-blind, placebo-controlled Phase 2 clinical trial in 47 patients with PWS, Millendo observed that administration of livoletide once daily was associated with a clinically meaningful improvement in hyperphagia, as well as a reduction in appetite. In a pre-specified analysis of 38 home-resident PWS patients from the Phase 2 trial, Millendo observed a larger and statistically significant decrease in hyperphagia following administration of livoletide as compared to placebo. Millendo expects to initiate a pivotal Phase 2b/3 clinical trial of livoletide in home-resident PWS patients in the first quarter of 2019, with results expected in the first half of 2020. Millendo is also developing nevanimibe (ATR-101) with a primary focus on treating patients with classic congenital adrenal hyperplasia, or CAH, a rare, monogenic adrenal disease that requires lifelong treatment with exogenous cortisol, often at high doses. These chronic high doses of cortisol can result in side effects that include diabetes, obesity, hypertension and psychological problems. When on suboptimal doses of cortisol, female CAH patients can experience male-pattern hair growth, male physical characteristics, infertility and menstrual irregularity, and male CAH patients can experience testicular atrophy, infertility and testicular tumors, making it difficult for physicians to appropriately treat CAH without causing adverse consequences. Millendo recently reported results from its Phase 2 clinical trial of nevanimibe in patients with CAH and initiated a Phase 2b trial in the third quarter of 2018, with results expected in the first half of 2020. Millendo is also investigating nevanimibe in a Phase 2 clinical trial for the treatment of patients with endogenous Cushing's syndrome, or CS, a rare endocrine disease characterized by excessive cortisol production from the adrenal glands.

Orion Merger Sub, Inc.

        Merger Sub is a wholly owned subsidiary of OvaScience, and was formed solely for the purposes of carrying out the Merger.

The Merger (see page 131)

        If the Merger is completed, Merger Sub will merge with and into Millendo, with Millendo surviving as a wholly owned subsidiary of OvaScience.

        At the Effective Time, each share of Millendo Common Stock outstanding immediately prior to the Effective Time (excluding shares held by stockholders who have exercised and perfected appraisal rights or dissenters' rights as more fully described in the section titled " The Merger—Appraisal Rights and Dissenters' Rights " in this proxy statement/prospectus/information statement, but including shares of Millendo Common Stock that will be issued pursuant to the Pre-Closing Financing as more fully described above under " Questions and Answers About the Merger—What is the Pre-Closing Financing? ") will be converted into the right to receive a number of shares of OvaScience Common Stock. The final Exchange Ratio will be determined pursuant to a formula described in more detail in the Merger Agreement and in this proxy statement/prospectus/information statement. As of the date of the execution of the Merger Agreement, it was estimated that immediately after the consummation of the Merger, based solely on the sample Exchange Ratio of 0.1173 set forth below, Millendo Securityholders would own approximately 82% of Fully Diluted Closing OvaScience Common Stock, and OvaScience Securityholders would own approximately 18% of Fully Diluted Closing OvaScience Common Stock, subject to adjustment of the Exchange Ratio as set forth in the Merger Agreement.

        OvaScience will assume all outstanding and unexercised Millendo Options and warrants to purchase Millendo Common Stock, and each such Millendo Options and warrants will be converted into an option or warrant, respectively, to purchase shares of OvaScience Common Stock, with the number of shares of OvaScience Common Stock subject to such option or warrant and the exercise price being appropriately adjusted to reflect the Exchange Ratio. The percentages set forth above assume that the initial estimate of the Exchange Ratio is not changed; however, the Exchange Ratio is subject to change as described in the section titled " The Merger Agreement—Merger Consideration " in this proxy statement/prospectus/information statement. The initial estimate of the Exchange Ratio set forth below assumed (i) that OvaScience would have between $40 million and $42 million in net cash immediately prior to Closing, (ii) a Pre-Closing Financing amount of $30.0 million, (iii) OvaScience outstanding shares and options as of the Closing would be equal to 3,721,460 (on a post reverse stock split basis), (iv) Millendo shares as of the Closing would be equal to 148,171,273 (on a fully-diluted, as-converted basis) and (v) a reverse stock split of every 10 shares of outstanding OvaScience Common Stock being combined and reclassified into one share of OvaScience Common Stock. Shares of Millendo Common Stock that are issued in the Pre-Closing Financing will be converted into shares of OvaScience Common Stock in the Merger. Accordingly, by approving Proposal No. 1 relating to the Merger, OvaScience Stockholders will also be approving the issuance of shares of OvaScience Common Stock to be issued in exchange for all shares of Millendo Common Stock that are sold in the Pre-Closing Financing.

        The Exchange Ratio is calculated using a formula intended to allocate a percentage of the combined organization to existing Millendo Securityholders. Based on the assumptions described above, the Exchange Ratio would have been equal to approximately 0.1173 post-reverse split shares of OvaScience Common Stock for each share of Millendo Common Stock, which Exchange Ratio is subject to change based on the amount of OvaScience net cash, changes in the capitalization of OvaScience or Millendo prior to the Closing and the amount raised by Millendo in the Pre-Closing Financing (and as a result, OvaScience Securityholders and Millendo Securityholders could own more or less of the combined organization).

        OvaScience currently estimates that, assuming for this purpose a closing date of November 30, 2018, (i) it will have approximately $37.6 million in net cash immediately prior to Closing, (ii) the Pre-Closing Financing amount will be approximately $29.5 million, (iii) OvaScience outstanding shares and options as of the Closing will be equal to 3,727,669 (on a post reverse stock split basis) and (iv) the Millendo shares as of the Closing on a fully diluted and as-converted basis will be equal to 152,629,595. Accordingly, it is currently estimated that the Exchange Ratio at Closing will be approximately 0.1070 and, based solely on such Exchange Ratio, at Closing: (a) Millendo Securityholders as of immediately prior to the Merger (not including the shares issued in the Pre-Closing Financing) will own approximately 67.1% of the Fully Diluted Closing OvaScience Common Stock (as defined below), (b) the shares issued in the Pre-Closing Financing to Millendo Securityholders as of immediately prior to the Merger will represent approximately 13.1% of the Fully Diluted Closing OvaScience Common Stock, (c) the OvaScience Securityholders as of immediately prior to the Merger (excluding for this purpose certain out-of-the-money OvaScience Options) will own approximately 18.6% of the Fully Diluted Closing OvaScience Common Stock, and (d) the shares available for issuance under the Millendo Plan as of immediately prior to the Merger will represent approximately 1.2% of the Fully Diluted Closing OvaScience Common Stock, in each case, subject to adjustment of the Exchange Ratio as set forth in the Merger Agreement and described herein. Additionally, it is anticipated that the Post-Closing Financing (as defined below) will close shortly after the Closing, which would dilute, and thereby reduce, each then existing OvaScience Securityholder's proportionate holdings. Assuming that the Post-Closing Financing closes immediately after the Merger as well as the other aforementioned assumptions, it is expected that (a) Millendo Securityholders as of immediately prior to the Merger (not including the shares issued in the Pre-Closing Financing) will own approximately 61.7% of the Fully Diluted Post-Financing OvaScience Common Stock (as defined below), (b) the shares issued in

the Pre-Closing Financing to Millendo Securityholders as of immediately prior to the Merger will represent approximately 12.0% of the Fully Diluted Post-Financing OvaScience Common Stock, (c) the OvaScience Securityholders as of immediately prior to the Merger (excluding for this purpose certain out-of-the-money OvaScience Options) will own approximately 17.1% of the Fully Diluted Post-Financing OvaScience Common Stock, (d) the shares issued in the Post-Closing Financing to the Post-Closing Financing Investor (as defined below) will represent approximately 8.1% of the Fully Diluted Post-Financing OvaScience Common Stock, and (e) the shares available for issuance under the Millendo Plan as of immediately prior to the Merger will represent approximately 1.1% of the Fully Diluted Post-Financing OvaScience Common Stock, in each case, subject to adjustment of the Exchange Ratio as set forth in the Merger Agreement and described herein. "Fully Diluted Closing OvaScience Common Stock" as used herein means (x) Orion Outstanding Shares (as defined in the Merger Agreement, which figure excludes the shares available for issuance under the 2012 Plan and a portion of the out-of-the-money OvaScience Options) plus (y) Company Outstanding Shares (as defined in the Merger Agreement, which figure includes the shares available for issuance under the Millendo Plan). "Fully Diluted Post-Financing OvaScience Common Stock" as used herein means (x) Orion Outstanding Shares (as defined in the Merger Agreement, which figure excludes the shares available for issuance under the 2012 Plan and a portion of the out-of-the-money OvaScience Options) plus (y) Company Outstanding Shares (as defined in the Merger Agreement, which figure includes the shares available for issuance under the Millendo Plan) plus (z) the Post-Closing Financing shares.

        Immediately prior to the Effective Time, each share of Millendo preferred stock and Millendo class-1 common stock will be converted into Millendo Common Stock. For a more complete description of the Merger and the Exchange Ratio please see the section titled " The Merger Agreement " in this proxy statement/prospectus/information statement.

        The Closing will occur no later than the second business day after the last of the conditions to the Merger has been satisfied or waived (other than those conditions that by their nature are to be satisfied at the Closing, but subject to the satisfaction or waiver of each such conditions), or at such other time as OvaScience and Millendo agree. OvaScience and Millendo anticipate that the consummation of the Merger will occur in the fourth quarter of 2018. However, because the Merger is subject to a number of conditions, neither OvaScience nor Millendo can predict exactly when the Closing will occur or if it will occur at all. After completion of the Merger, assuming that OvaScience receives the required stockholder approval of Proposal No. 3, OvaScience will be renamed "Millendo Therapeutics, Inc."

Reasons for the Merger

        Following the Merger, the combined organization will be a late-stage biopharmaceutical company focused on developing novel treatments for orphan endocrine diseases where current therapies do not exist or are insufficient. OvaScience and Millendo believe that the combined organization will have the following potential advantages:

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Late-Stage Specialty Biopharmaceutical Company.   Millendo is focused on developing novel treatments for orphan endocrine diseases where current therapies do not exist or are insufficient. Millendo is currently advancing two product candidates for three indications: livoletide for the treatment of Prader-Willi syndrome and nevanimibe for the treatment of classic congenital adrenal hyperplasia and Cushing's syndrome.



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Management Team.   It is expected that the combined organization will be led by the experienced senior management from Millendo and a board of directors with representation from each of OvaScience and Millendo.



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Cash Resources.   The combined organization is expected to have approximately $81.1 million in cash, cash equivalents and short-term investments at the Closing, which OvaScience and Millendo believe is sufficient to enable Millendo to implement its near-term business plans. The

cash, cash equivalents and short-term investments amount assumes that OvaScience's transaction costs and other obligations will be paid at Closing, other than certain severance payment amounts, which may be paid out up to one year after Closing.

        Each of the boards of directors of OvaScience and Millendo also considered other reasons for the Merger, as described herein. For example, the OvaScience Board of Directors considered, among other things:

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the strategic alternatives to the Merger available to OvaScience, including the discussions that OvaScience's management and the OvaScience Board of Directors previously conducted with other potential merger partners;



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the risks and delays associated with, and uncertain value and costs to OvaScience Stockholders of, liquidating OvaScience, including the uncertainties of continuing cash burn while contingent liabilities are resolved, uncertainty of timing of release of cash until contingent liabilities are resolved, and the risks associated with being a shell company prior to cash distribution, including the risks associated with delisting;



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the risks of continuing to operate OvaScience on a stand-alone basis, including the need to rebuild infrastructure and management to continue its operations; and



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the opportunity as a result of the Merger for OvaScience Stockholders to participate in the potential value of Millendo's product candidate portfolio and the potential growth of the combined organization following the Merger.

        In addition, the Millendo Board of Directors approved the Merger based on a number of factors, including the following:

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the process undertaken by the Millendo Board of Directors and management to ascertain actionable alternatives, including partnering transactions and including possibly the sale of the company and whether this was an appropriate juncture, in light of the progress achieved and additional challenges faced by Millendo, to undertake such a process;



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the possible alternatives to the Merger, the range of possible benefits to the Millendo Stockholders of such alternatives and the timing and the likelihood of accomplishing the goal of any of such alternatives;



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the financial condition, historical results of operations and business and strategic objectives Millendo, as well as the risks involved in achieving those objectives;



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the amount and form of consideration to be received by the Millendo Stockholders in the Merger pursuant to the Merger Agreement taking into account the relative interests of the various classes of stock of Millendo and the contractual rights afforded to holders of the Preferred Stock of the Company (including as to whether any alternatives to the Merger would reasonably likely be achievable and derive more value across such classes of stock);



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the expectation that the Merger will be treated as a tax-deferred reorganization for U.S. federal income tax purposes; and



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the proposed timing of the interim period between the signing of the Merger Agreement and the expected Closing and whether it is advisable to proceed given current economic, industry and market conditions.

Opinions of the OvaScience Financial Advisor as of August 8, 2018 (see page 143)

        The OvaScience Board of Directors engaged Ladenburg Thalmann to provide strategic advisory and investment banking services in connection with evaluating and considering various strategic

alternatives, and ultimately requested that Ladenburg Thalmann render an opinion as to whether the Exchange Ratio was fair, from a financial point of view, to the OvaScience Stockholders. At the August 8, 2018 meeting of the OvaScience Board of Directors, Ladenburg Thalmann rendered its oral opinion, subsequently confirmed by delivery of its written opinion dated August 8, 2018, to the OvaScience Board of Directors. On October 26, 2018, at the request of the OvaScience Board of Directors and in light of the changes proposed to the valuation of Millendo and OvaScience as set forth in the Second Amendment to the Merger Agreement, Ladenburg Thalmann rendered a revised oral opinion, subsequently confirmed by delivery of the written opinion dated October 26, 2018, to the OvaScience Board of Directors, that the Exchange Ratio was fair, from a financial point of view, to the OvaScience Stockholders as of the date of such opinion and based upon the various assumptions, qualifications and limitations set forth therein.

         The full texts of Ladenburg Thalmann's written opinions, which set forth the procedures followed, assumptions made, matters considered, and qualifications and limitations of the review undertaken in connection with the opinions, are attached to this proxy statement/prospectus/information statement as Annex B-1 and B-2 and are incorporated by reference in their entirety to this proxy statement/prospectus/information statement. Ladenburg Thalmann's opinions were intended solely for the benefit and use of the OvaScience Board of Directors (in its capacity as such) in connection with its consideration of the Merger. Ladenburg Thalmann's opinions were not intended to be used for any other purpose without Ladenburg Thalmann's prior written consent in each instance, except as expressly provided for in the engagement letter between OvaScience and Ladenburg Thalmann. Ladenburg Thalmann has consented to the use of Ladenburg Thalmann's opinions in this proxy statement/prospectus/information statement. Ladenburg Thalmann's opinions did not address OvaScience's underlying business decision to enter into the Merger Agreement or complete the Merger or the merits of the Merger as compared to any alternative transactions that were or may be available to OvaScience, and did not constitute a recommendation to the OvaScience Board of Directors or to any OvaScience Stockholder or Millendo Stockholder as to how such stockholder should vote with respect to the Merger or otherwise.

Overview of the Merger Agreement

Merger Consideration (see page 200)

        At the Effective Time, each outstanding share of common stock of Millendo will be converted into the right to receive a number of shares of OvaScience Common Stock.

        The Merger Agreement does not provide for an adjustment to the total number of shares of OvaScience Common Stock that Millendo Stockholders will be entitled to receive for changes in the market price of OvaScience Common Stock. Accordingly, the market value of the shares of OvaScience Common Stock issued pursuant to the Merger will depend on the market value of the shares of OvaScience Common Stock at the time the Merger closes, and could vary significantly from the market value on the date of this proxy statement/prospectus/information statement.

Treatment of Millendo Options (see page 201)

        Pursuant to the Merger Agreement, at the Effective Time, each option to purchase shares of Millendo Common Stock (a "Millendo Option") that is outstanding and unexercised immediately prior to the Effective Time granted under the Millendo 2012 Equity Incentive Plan (the "Millendo Plan") or otherwise, whether or not vested, will be, along with the Millendo Plan, assumed by OvaScience and will become an option to purchase solely that number of shares of OvaScience Common Stock equal to the product obtained by multiplying (i) the number of shares of Millendo Common Stock that were subject to such Millendo Option immediately prior to the Effective Time by (ii) the Exchange Ratio, and rounding the resulting number down to the nearest whole number of shares of OvaScience

Common Stock. The per share exercise price for OvaScience Common Stock issuable upon exercise of each Millendo Option assumed by OvaScience shall be determined by dividing (a) the per share exercise price of Millendo Common Stock subject to such Millendo Option, as in effect immediately prior to the Effective Time, by (b) the Exchange Ratio, and rounding the resulting exercise price up to the nearest whole cent. Any restriction on the exercise of any Millendo Option assumed by OvaScience will continue in full force and effect and the term, exercisability, vesting schedule and other provisions of such Millendo Option shall otherwise remain unchanged.

Treatment of Millendo Warrants (see page 201)

        Pursuant to the Merger Agreement, at the Effective Time, each warrant to purchase shares of Millendo Common Stock (a "Millendo Warrant") that is outstanding and unexercised immediately prior to the Effective Time, whether or not vested, will be converted into and become a warrant to purchase (and OvaScience shall assume each such Millendo Warrant in accordance with its terms) solely that number of shares of OvaScience Common Stock equal to the product obtained by multiplying (i) the number of shares of Millendo Common Stock that were subject to such Millendo Warrant immediately prior to the Effective Time by (ii) the Exchange Ratio, and rounding the resulting number down to the nearest whole number of shares of OvaScience Common Stock. The per share exercise price for OvaScience Common Stock issuable upon exercise of each Millendo Warrant assumed by OvaScience shall be determined by dividing (a) the per share exercise price of Millendo Common Stock subject to such Millendo Warrant, as in effect immediately prior to the Effective Time, by (b) the Exchange Ratio, and rounding the resulting exercise price up to the nearest whole cent. Any restriction on the exercise of any Millendo Warrant assumed by OvaScience will continue in full force and effect and the term, exercisability, vesting schedule and other provisions of such Millendo Warrant shall otherwise remain unchanged. Immediately prior to the Effective Time, each share of Millendo preferred stock and Millendo class-1 common stock will be converted into Millendo Common Stock.

Treatment of OvaScience Options (see page 201)

        Prior to the Closing, OvaScience's board of directors will adopt appropriate resolutions and take all other actions necessary and appropriate to provide that the vesting of each unexpired and unexercised option to purchase shares of OvaScience Common Stock, whether vested or unvested, will be accelerated in full effective as of immediately prior to the Effective Time, and each such option will survive the Closing and remain outstanding in accordance with its terms. The number of shares of OvaScience Common Stock underlying such options and the exercise prices for such options will be appropriately adjusted to reflect the proposed Reverse Stock Split to be implemented prior to the Closing.

Conditions to Completion of the Merger (see page 215)

        To consummate the Merger, OvaScience Stockholders must approve (a) the Merger Agreement and the transactions contemplated thereby, including the Merger and the issuance of shares of OvaScience Common Stock to Millendo Stockholders in the Merger, and (b) an amendment to the restated certificate of incorporation of OvaScience effecting the Reverse Stock Split. Additionally, Millendo Stockholders must adopt the Merger Agreement thereby approving the Merger and the other transactions contemplated by the Merger Agreement. In addition to obtaining such stockholder approvals and obtaining appropriate regulatory approvals, each of the other closing conditions set forth in the Merger Agreement must be satisfied or waived.

No Solicitation (see page 209)

        Each of OvaScience and Millendo have agreed that, except as described below, OvaScience and Millendo and any of their respective subsidiaries will not, nor will either party or any of its subsidiaries

authorize any of the officers, directors, employees, representatives, affiliates, advisors or agents, directly or indirectly:

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initiate, solicit, seek or knowingly encourage or support any inquiries, proposals or offers that constitute or may reasonably be expected to lead to any competing proposal;



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engage or participate in, or knowingly facilitate, any discussions or negotiations regarding, or furnish any nonpublic information to any person in connection with, any inquiries, proposals or offers that constitute, or may reasonably be expected to lead to, a competing proposal;



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enter into any letter of intent, agreement in principle or other similar type of agreement relating to a competing proposal, or enter into any agreement or agreement in principle requiring either OvaScience or Millendo, as the case may be, to abandon, terminate or fail to complete the Merger; or



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resolve, propose or agree to do any of the foregoing.

        However, prior to the approval of the proposals relating to the Merger set forth in this proxy statement at the meeting of the OvaScience Stockholders or by written consent of Millendo Stockholders (unless, in either case, the Merger Agreement is earlier terminated), as the case may be, either OvaScience or Millendo may, after providing written notice to the other party, furnish nonpublic information to and engage in discussions or negotiations with any third-party that makes an unsolicited bona fide written competing proposal (as described in the Merger Agreement) that its board of directors in good faith, after consultation with its outside legal counsel and financial advisors, has determined constitutes or would reasonably be expected to lead to a superior competing proposal, only if:

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such party receives from such third-party an executed confidentiality agreement the terms of which are not less restrictive to the third-party than those contained in the confidentiality agreement between OvaScience and Millendo;



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such party receiving the competing proposal contemporaneously supplies to the other party (OvaScience or Millendo, as the case may be) any nonpublic information or access to any such nonpublic information granted to such third-party to the extent it had not been previously provided or made available;



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such party has not breached the no solicitation provisions of the Merger Agreement; and



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the board of directors of OvaScience or Millendo, as the case may be, determines in good faith, after consultation with its outside legal counsel and its financial advisors that taking such actions would be required to comply with the fiduciary duties of the board of directors under applicable laws.

Termination of the Merger Agreement (see page 220)

        Either OvaScience or Millendo can terminate the Merger Agreement under certain circumstances, which would prevent the Merger from being consummated.

Termination Fee (see page 222)

        The Merger Agreement contains certain termination rights for both OvaScience and Millendo, and further provides that, upon termination of the Merger Agreement under specified circumstances, either party may be required to pay the other party a termination fee of $3.0 million, or in some circumstances reimburse the other party's expenses up to a maximum of $1.0 million. In addition, in certain specified circumstances, Millendo may be required to pay OvaScience a termination fee of $15.0 million.

Voting Agreements (see page 225)

        Certain Millendo Stockholders are party to a voting agreement with OvaScience and Millendo pursuant to which, among other things, each of these stockholders agreed, solely in his, her or its capacity as a Millendo Stockholder, to vote all of his, her or its shares of Millendo Capital Stock in favor of the adoption of the Merger Agreement and the approval of any transaction proposed under the Merger Agreement and any other proposal included in the written consent presented to Millendo's Stockholders in connection with, or related to, the consummation of the Merger for which the Board of Directors of Millendo has recommended that Millendo's Stockholders vote in favor.

        As of September 1, 2018, the Millendo Stockholders that are party to a voting agreement with OvaScience owned approximately 73% of the outstanding shares of Millendo Capital Stock. Following the effectiveness of the registration statement of which this proxy statement/prospectus/information statement is a part, Millendo Stockholders holding a sufficient number of shares of Millendo Capital Stock to adopt the Merger Agreement and thereby approve the Merger will, pursuant to their voting agreements, execute written consents providing for such adoption and approval.

        Certain OvaScience Stockholders are party to a voting agreement with Millendo pursuant to which, among other things, each of these stockholders agreed, solely in his, her or its capacity as a OvaScience Stockholder, to vote all of his, her or its shares of OvaScience Common Stock in favor of approval of (i) the Merger Agreement and the transactions contemplated thereby, including the Merger and the issuance of shares of OvaScience Common Stock to Millendo Stockholders, (ii) an amendment to the restated certificate of incorporation of OvaScience to effect the Reverse Stock Split, (iii) an amendment to the restated certificate of incorporation of OvaScience to effect the OvaScience Name Change, (iv) any proposal to adjourn or postpone the meeting to a later date, if there are not sufficient votes for the approval of the Merger and other matters to be approved on date of the Special Meeting, and (v) any other proposal included in this proxy statement/prospectus/information statement in connection with, or related to, the consummation of the Merger for which the Board of Directors of OvaScience has recommended that the stockholders of OvaScience vote in favor.

        The OvaScience Stockholders that are party to a voting agreement with Millendo beneficially owned approximately 5% of the outstanding shares of OvaScience Common Stock as of September 1, 2018. The parties to the voting agreements with Millendo include the executive officers and directors of OvaScience and certain OvaScience Stockholders.

        The voting agreements are discussed in greater detail in the section titled " Agreements Related to the Merger—Voting Agreements " in this proxy statement/prospectus/information statement.

Lock-up Agreements (see page 226)

        As a condition to the Closing, certain OvaScience Stockholders and Millendo Stockholders have entered into lock-up agreements, pursuant to which such parties have agreed not to, except in limited circumstances, sell or transfer, or engage in swap or similar transactions with respect to, shares of OvaScience Common Stock, including, as applicable, shares received in the Merger and issuable upon exercise of certain options, in each case from the Closing until the date that is 180 days from the Closing.

        As of September 1, 2018, OvaScience Stockholders who have executed lock-up agreements beneficially owned in the aggregate approximately 3% of the outstanding shares of OvaScience Common Stock.

        Millendo Stockholders who have executed lock-up agreements as of September 1, 2018 owned in the aggregate approximately 97% of the outstanding shares of Millendo Capital Stock on an as converted into common stock basis.

Management Following the Merger (see page 342)

        Effective as of the Closing, OvaScience's officers are expected to include:

Interests of Certain Directors, Officers and Affiliates of OvaScience and Millendo (see pages 165 and 182)

        In considering the recommendation of the OvaScience Board of Directors with respect to issuing shares of OvaScience Common Stock pursuant to the Merger Agreement and the other matters to be acted upon by OvaScience Stockholders at the Special Meeting, OvaScience Stockholders should be aware that certain members of the OvaScience Board of Directors and executive officers of OvaScience have interests in the Merger that may be different from, or in addition to, interests they have as OvaScience Stockholders. For example, OvaScience has entered into certain retention agreements with its current executive officers that may result in the receipt by such executive officers of cash severance payments and other benefits with a total value of approximately $2.3 million (collectively, not individually, and excluding the value of any accelerated vesting of OvaScience Options) and the accelerated vesting of OvaScience Options held by those officers, based on data available as of September 1, 2018 and assuming a covered termination of employment of each executive officer's employment as of such date.

        As of September 1, 2018, the directors and executive officers of OvaScience beneficially owned, in the aggregate, approximately 8.0% of the outstanding shares of OvaScience Common Stock. Each of OvaScience's officers and directors, as well as an affiliate of one of OvaScience's directors, have entered into voting agreements in connection with the Merger. The voting agreements are discussed in greater detail in the section titled " Agreements Related to the Merger—Voting Agreements " in this proxy statement/prospectus/information statement.

        In considering the recommendation of the Millendo Board of Directors with respect to approving the Merger and related transactions, Millendo Stockholders should be aware that certain members of the Millendo Board of Directors and certain executive officers of Millendo have interests in the Merger that may be different from, or in addition to, interests they have as Millendo Stockholders. For example, Millendo's executive officers have options, subject to vesting, to purchase shares of Millendo Common Stock, which will convert into options to purchase a number of shares of OvaScience Common Stock determined by the Exchange Ratio, rounding any resulting fractional shares down to the nearest whole share, certain of Millendo's directors and executive officers are expected to become directors and executive officers of the combined organization upon the Closing and all of Millendo's directors and executive officers are entitled to certain indemnification and liability insurance coverage pursuant to the terms of the Merger Agreement. For more information, please see the section titled " The Merger—Interests of the Millendo Directors and Executive Officers in the Merger. "

Material U.S. Federal Income Tax Consequences of the Merger (see page 192)

        As discussed in detail in the section titled "The Merger—Material U.S. Federal Income Tax Consequences of the Merger" in this proxy statement/prospectus/information statement, OvaScience and Millendo intend the Merger to qualify as a reorganization within the meaning of Section 368(a) of the Code. In general, and subject to the qualifications and limitations set forth in the section titled " The Merger—Material U.S. Federal Income Tax Consequences of the Merger " in this proxy statement/

prospectus/information statement, if the Merger qualifies as a "reorganization" within the meaning of Section 368(a) of the Code, the material tax consequences to a U.S. holder of Millendo Capital Stock should be as follows:

•

such Millendo Stockholder will not recognize gain or loss upon the exchange of Millendo Capital Stock for OvaScience Common Stock pursuant to the Merger, except to the extent of cash received in lieu of a fractional share of OvaScience Common Stock as described below;



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such Millendo Stockholder who receives cash in lieu of a fractional share of OvaScience Common Stock in the Merger will recognize capital gain or loss in an amount equal to the difference between the amount of cash received in lieu of a fractional share and the stockholder's tax basis allocable to such fractional share;



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such Millendo Stockholder's aggregate tax basis for the shares of OvaScience Common Stock received in the Merger (including any fractional share interest for which cash is received) will equal the stockholder's aggregate tax basis in the shares of Millendo Capital Stock surrendered in the Merger; and



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the holding period of the shares of OvaScience Common Stock received by such Millendo Stockholder in the Merger will include the holding period of the shares of Millendo Capital Stock surrendered in exchange therefor.

        If the Merger is not treated as a reorganization within the meaning of Section 368(a) of the Code, then each U.S. holder generally will be treated as exchanging its shares of Millendo Capital Stock in a fully-taxable transaction in exchange for shares of OvaScience Common Stock. Millendo Stockholders will generally recognize gain or loss in such exchange equal to the amount that such Millendo Stockholder's adjusted tax basis in the shares of Millendo Capital Stock surrendered is less or more than the fair market value of the shares of OvaScience Common Stock (and cash in lieu of a fractional share) received in exchange therefor. Determining the actual tax consequences of the Merger to you may be complex and will depend on the facts of your own situation. You should consult your tax advisors to fully understand the tax consequences to you of the Merger, including estate, gift, state, local or non-U.S. tax consequences of the Merger.

Risk Factors (see page 34)

        Both OvaScience and Millendo are subject to various risks associated with their businesses and their industries. In addition, the Merger poses a number of risks to each company and its respective stockholders, including the possibility that the Merger may not be completed and the following risks:

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the Exchange Ratio is not adjustable based on the market price of OvaScience Common Stock, so the merger consideration at the Closing may have a greater or lesser value than at the time the Merger Agreement was signed;



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failure to complete the Merger may result in OvaScience or Millendo paying a termination fee or expenses to the other and could harm the per share price of OvaScience Common Stock and future business and operations of each company;



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the Merger may be completed even though material adverse changes may result solely from the announcement of the Merger, general economic or political conditions or conditions generally affecting the industries in which OvaScience and Millendo operate and other causes;



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some OvaScience and Millendo officers and directors have interests that are different from or in addition to those considered by stockholders of OvaScience and Millendo and which may influence them to support or approve the Merger;



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the market price of OvaScience Common Stock may decline as a result of the Merger;

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OvaScience Stockholders and Millendo Stockholders may not realize a benefit from the Merger commensurate with the ownership dilution they will experience in connection with the Merger;



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during the pendency of the Merger, OvaScience and Millendo may not be able to enter into a business combination with another party under certain circumstances because of restrictions in the Merger Agreement, which could adversely affect their respective businesses;



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certain provisions of the Merger Agreement may discourage third parties from submitting alternative takeover proposals, including proposals that may be superior to the arrangements contemplated by the Merger Agreement;



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because the lack of a public market for shares of Millendo Capital Stock makes it difficult to evaluate the fairness of the Merger, the Millendo Stockholders may receive consideration in the Merger that is less than the fair market value of the shares of Millendo Capital Stock and/or OvaScience may pay more than the fair market value of the shares of Millendo Capital Stock; and



•

if the conditions to the Merger are not met, the Merger will not occur.

        These risks and other risks are discussed in greater detail under the section titled " Risk Factors " in this proxy statement/prospectus/information statement. OvaScience and Millendo both encourage you to read and consider all of these risks carefully.

Regulatory Approvals (see page 192)

        In the United States, OvaScience must comply with applicable federal and state securities laws and the rules and regulations of The Nasdaq Stock Market LLC ("Nasdaq") in connection with the issuance of shares of OvaScience Common Stock and the filing of this proxy statement/prospectus/information statement with the SEC.

Nasdaq Capital Market Listing (see page 195)

        Prior to consummation of the Merger, OvaScience has filed an initial listing application with The Nasdaq Capital Market pursuant to Nasdaq's rules for companies conducting a business combination that results in a change of control. If such application is accepted, OvaScience anticipates that shares of OvaScience Common Stock will be listed on The Nasdaq Capital Market following the Closing under OvaScience's new name, "Millendo Therapeutics, Inc.," with the trading symbol "MLND."

Anticipated Accounting Treatment (see page 196)

        The Merger is expected be treated by OvaScience as a reverse merger and accounted for as a reverse recapitalization in accordance with U.S. generally accepted accounting principles ("U.S. GAAP"). For accounting purposes, Millendo is considered to be acquiring OvaScience in the Merger.

Appraisal Rights and Dissenters' Rights (see page 196)

        Holders of shares of OvaScience Common Stock are not entitled to appraisal rights in connection with the Merger. Millendo Stockholders are entitled to appraisal rights in connection with the Merger under Delaware law. For more information about such rights, see the provisions of Section 262 of the General Corporation Law of the State of Delaware (the "DGCL") attached hereto as Annex C , and the section titled " The Merger—Appraisal Rights and Dissenters' Rights " in this proxy statement/prospectus/information statement.

Comparison of Stockholder Rights (see page 386)

        Both OvaScience and Millendo are incorporated under the laws of the State of Delaware and, accordingly, the rights of the stockholders of each are currently, and will continue to be, governed by the DGCL. If the Merger is completed, Millendo Stockholders will become OvaScience Stockholders, and their rights will be governed by the DGCL, the amended and restated bylaws of OvaScience and, assuming Proposals No. 2 and 3 are approved by OvaScience Stockholders at the Special Meeting, the restated certificate of incorporation of OvaScience. The rights of OvaScience Stockholders contained in the restated certificate of incorporation and amended and restated bylaws of OvaScience differ from the rights of Millendo Stockholders under the amended and restated certificate of incorporation and amended bylaws of Millendo, as more fully described under the section titled " Comparison of Rights of Holders of OvaScience Stock and Millendo Stock " in this proxy statement/prospectus/information statement.