SUMMARY

This summary highlights information contained elsewhere in this proxy statement/prospectus. This summary may not contain all the information that may be important to you, and you are urged to read this entire proxy statement/prospectus carefully, including the attached annexes, and the other documents to which this proxy statement/prospectus refers or which are incorporated by reference herein in order for you to fully understand the proposed Merger. See also the sections entitled “Where You Can Find More Information” and “Incorporation of Certain Documents by Reference.”

Information about the Companies

Mereo BioPharma Group plc

Mereo is a biopharmaceutical company focused on the development and commercialization of innovative therapeutics that aim to improve outcomes for patients with rare diseases. Mereo’s portfolio consists of four clinical-stage product candidates, each of which Mereo acquired from large pharmaceutical companies. Mereo is developing BPS-804 for the treatment of osteogenesis imperfecta (“OI”), MPH-966 for the treatment of severe alpha-1 antitrypsin deficiency (“AATD”), BCT-197 for the treatment of acute exacerbations of chronic obstructive pulmonary disease (“AECOPD”), and BGS-649 for the treatment of hypogonadotropic hypogonadism (“HH”) in obese men. Each of Mereo’s product candidates has generated positive clinical data for such product candidate’s target indication or for a related indication. Mereo believes its portfolio is well diversified because each of its product candidates employs a different mechanism of action and targets a separate indication. Mereo intends to develop and directly commercialize its rare disease product candidates. For its specialty disease product candidates, Mereo intends to seek strategic relationships for further clinical development and commercialization.

Mereo’s strategy is to selectively acquire product candidates that have already received significant investment from pharmaceutical companies and that have substantial pre-clinical, clinical, and manufacturing data packages. Since Mereo’s formation in March 2015, it has successfully executed on this strategy by acquiring product candidates from Novartis Pharma AG (“Novartis”) and AstraZeneca AB (“AstraZeneca”). Mereo has commenced or completed large, randomized, placebo-controlled Phase 2 clinical trials for all of its product candidates.

The principal executive offices of Mereo are located at 4 ^th Floor, 1 Cavendish Place, London, W1G 0QF, United Kingdom; its telephone number is +44 333 023 7300; and its website is www.mereobiopharma.com. Information on Mereo’s website is not incorporated by reference into or otherwise part of this proxy statement/prospectus.

Mereo MergerCo One Inc.

Merger Sub is a wholly-owned indirect subsidiary of Mereo and was formed on December 3, 2018 exclusively for the purpose of effecting the Merger. Merger Sub has not carried on any activities to date, except for activities incidental to its formation and activities undertaken in connection with the Merger. Merger Sub’s separate corporate existence will cease upon the consummation of the Merger and OncoMed will continue as the surviving corporation.

The address and telephone number for Merger Sub’s principal executive offices are the same as Mereo’s.

OncoMed Pharmaceuticals, Inc.

OncoMed is a clinical-stage biopharmaceutical company focused on discovering and developing novel therapeutics that address the fundamental biology driving cancer’s growth, resistance, recurrence and metastasis. OncoMed has three anti-cancer therapeutic candidates currently in clinical development, navicixizumab (anti-DLL4/VEGF, OMP-305B83), etigilimab (anti-TIGIT, OMP-313M32), and GITRL-Fc (OMP-336B11). Each of these therapeutic candidates was discovered by OncoMed scientists. OncoMed is currently conducting a Phase 1b clinical trial of its first therapeutic candidate, navicixizumab, in combination with standard chemotherapy regimens in patients with platinum-resistant ovarian cancer. OncoMed is also conducting a Phase 1a/b clinical trial of its second therapeutic candidate, etigilimab. Etigilimab is being tested as a single agent in patients with advanced or metastatic solid tumors in the Phase 1a portion of the Phase 1a/b trial and in combination with nivolumab (anti-PD1) in the Phase 1b portion of the trial. OncoMed has a strategic collaboration with Celgene Corporation regarding the etigilimab program. GITRL-Fc, OncoMed’s third therapeutic candidate, is currently in a Phase 1a clinical trial in patients with advanced or metastatic solid tumors, although OncoMed does not plan to advance GITRL-Fc beyond Phase 1a. Data for OncoMed’s two lead therapeutic candidates, navicixizumab and etigilimab, are being gathered to inform the advancement of these therapeutic candidates into later stage clinical trials independently or with potential or existing partners, with the goal of ultimately obtaining regulatory approvals and improving patient outcomes.

The principal trading market for shares of OncoMed common stock (Nasdaq: OMED) is Nasdaq. The principal executive offices of OncoMed are located at 800 Chesapeake Drive, Redwood City, California 94063; its telephone number is (650) 995-8200; and its website is www.oncomed.com. Information on OncoMed’s website is not incorporated by reference into or otherwise part of this proxy statement/prospectus.

Summary of the Merger (page 114)

Subject to the terms and conditions of the Merger Agreement, Merger Sub, a wholly-owned indirect subsidiary of Mereo, will be merged with and into OncoMed, and OncoMed will continue as the surviving corporation in the Merger and a wholly-owned indirect subsidiary of Mereo. At the Effective Time, OncoMed’s restated certificate of incorporation will be amended and restated in the form prescribed in the Merger Agreement, and will be the certificate of incorporation of the surviving corporation from and after the Effective Time.

Merger Consideration (page 151)

Subject to the terms and conditions of the Merger Agreement, at the Effective Time, OncoMed stockholders will receive, in exchange for each outstanding share of OncoMed common stock owned immediately prior to completion of the Merger (except for any dissenting shares): (1) a number of Mereo ADSs determined by reference to the Exchange Ratio described below, and (2) one CVR, representing the right to receive contingent payments if specified milestones are achieved within agreed time periods, subject to and in accordance with the terms and conditions of the CVR Agreement, to be entered into at or prior to the Effective Time by and among Computershare Inc., as rights agent, and Mereo.

Under the exchange ratio formula set forth in the Merger Agreement, as of immediately following the Effective Time, former OncoMed stockholders are expected to own approximately 25% of the outstanding equity interests in the Combined Company on an undiluted basis, subject to adjustment for the net cash held by OncoMed at the time of the closing of the Merger. The number of Mereo Shares underlying Mereo ADSs issuable to former OncoMed stockholders in the Merger is also subject to the

limitation that the number of Mereo Shares to be allotted and issued by Mereo (and the corresponding number of Mereo ADSs to be issued to holders of OncoMed common stock) as Share Consideration or pursuant to the CVR Agreement will not, in the aggregate, exceed 66.67% of the Mereo Shares issued and outstanding immediately prior to the Effective Time (approximately 40% of the share capital of the Combined Company), as described further in “The Merger Agreement—Merger Consideration.” No fractional Mereo ADSs or CVRs will be issued in connection with the Merger. Any fractional Mereo ADSs or CVRs will be rounded down to the nearest whole Mereo ADS or CVR, as applicable, with no cash being paid to compensate for such rounding. Mereo intends to apply to list the Mereo ADSs on Nasdaq.

Because the number of Mereo ADSs to be exchanged for each share of OncoMed common stock will be adjusted based on the net cash held by OncoMed at the time of the closing of the Merger and will be unaffected by any increase or decrease in exchange rates or in the share price of Mereo Shares between now and the closing of the Merger, the notional value of the Merger Consideration and the exact number of Mereo ADSs that will be issued to OncoMed stockholders as of the date of the OncoMed Special Meeting and as of the date of the closing of the Merger cannot be determined with precision in advance of the Effective Time.

The CVRs (page 175)

The CVRs will be governed by the terms of the CVR Agreement, which will be entered into at or prior to the Effective Time by Mereo and Computershare, Inc., as rights agent.

The CVRs represent the non-transferable contractual right to receive certain stock and cash payments from Mereo if specified milestones are achieved within agreed time periods. Each share of OncoMed common stock outstanding immediately prior to the Effective Time shall be converted into the right to receive one CVR, in addition to the Share Consideration.

The CVR milestones relate to OncoMed’s etigilimab (anti-TIGIT, OMP-313M32) and navicixizumab (anti-DLL4/VEGF, OMP-305B83) therapeutic candidates. The contingent payments become payable to the rights agent, for subsequent distribution to the holders of the CVRs, upon the achievement of the milestones as follows:

The TIGIT Milestone

A payment, in the form of Mereo ADSs, will be made to CVR holders if, following the Effective Time but prior to December 31, 2019, the following milestone is achieved:

If the TIGIT Milestone is achieved, holders of CVRs would be entitled to receive a number of Mereo ADSs equal to (x) the amount of the cash payment actually received by OncoMed upon the Celgene Option Exercise, net of any tax and other reasonable expenses, divided by (y) the volume-weighted average price per Mereo ADS for the ten trading day period immediately following the date of the announcement by Mereo of the receipt of such cash payment. The TIGIT Milestone payment is subject to the Share Consideration Cap, such that the number of Mereo Shares underlying the Mereo ADSs to be issued pursuant to the CVR Agreement, when aggregated with the number of Mereo Shares underlying the Mereo ADSs issued as Share Consideration pursuant to the Merger Agreement, cannot exceed the Share Consideration Cap.

In the event that the cash payment payable by Celgene pursuant to the Celgene Option Exercise is actually received by OncoMed prior to the Effective Time, OncoMed stockholders will receive value for such cash payment in the form of additional Mereo ADSs issued at the Effective Time pursuant to the application of the Exchange Ratio set forth in the Merger Agreement. In such event, the CVR Agreement will be amended to delete the TIGIT Milestone and all related payment mechanics.

The NAVI Milestones

A cash payment will be made to CVR holders if, within eighteen months following the closing of the Merger, Mereo or any of its subsidiaries enters into a definitive agreement with one or more third parties regarding the navicixizumab products and, within five years of the closing of the Merger, Mereo or any of its subsidiaries actually receives certain eligible cash milestone payments.

If a NAVI Milestone is achieved, holders of CVRs would be entitled to receive an amount in cash equal to 70% of the amount of such eligible cash milestone payment, net of any tax and other reasonable expenses. The NAVI milestone payments are subject to a cash consideration cap, pursuant to which the aggregate principal amount of all cash payments made to holders of CVRs by Mereo shall in no case exceed $79.7 million.

The CVRs may not be sold, assigned, transferred, pledged or disposed of in any other manner, in whole or in part, other than in the limited circumstances specified in the CVR Agreement. In addition, the CVRs (i) will not be evidenced by a certificate or other instrument, (ii) will not have any voting or dividend rights and (iii) will not represent any equity or ownership interest in Mereo or any of its subsidiaries or in the surviving corporation. No interest will accrue on any amounts payable in respect of the CVRs.

Mereo’s obligation to make the CVR payment, if any becomes due, is neither secured nor guaranteed. Mereo’s obligation to make the CVR payment, if any becomes due, is an unsecured general obligation of Mereo and is not guaranteed by Mereo or any of its affiliates.

Treatment of OncoMed Options and OncoMed Units (page 154)

OncoMed Options

At or immediately prior to the Effective Time, each outstanding and unexercised OncoMed Option will be automatically canceled and converted into the right to receive (i) the excess, if any, of the Merger Consideration over the applicable exercise price of such canceled OncoMed Option, multiplied by (ii) the number of shares of OncoMed common stock subject to such OncoMed Option immediately prior to the Effective Time, provided that no fractional Mereo ADSs or CVRs will be issued. Each OncoMed Option that has a per-share exercise price that is higher than the Merger Consideration shall be canceled at the Effective Time for no consideration. No OncoMed Options will remain outstanding following the consummation of the Merger.

OncoMed Units

Immediately prior to the Effective Time and contingent on the occurrence of the Closing, each OncoMed Unit will be canceled and the holders thereof will be entitled to receive, immediately prior to the Effective Time, the number of shares of OncoMed common stock that were subject to such OncoMed Units (with the tax withholding obligations of each holder of such OncoMed Units being satisfied by OncoMed withholding from issuance that number of shares of OncoMed common stock calculated by multiplying the maximum statutory withholding rate for such holder in connection with such issuance by the number of shares of OncoMed common stock to be issued, rounding up to the

nearest whole share, and remitting such withholding in cash to the appropriate tax authority) and such stockholders will thereafter be entitled to receive the Merger Consideration in respect of these shares of OncoMed common stock. No OncoMed Units will remain outstanding following the consummation of the Merger.

Comparative per Share Market Price and Dividend Information (page 37)

Mereo Shares are quoted in pence on AIM under the symbol “MPH.” Shares of OncoMed’s common stock are listed for trading in U.S. dollars on Nasdaq under the symbol “OMED.” The following table sets forth the closing sales prices of a Mereo Share (as reported on AIM in pence) and of OncoMed common stock (as reported on Nasdaq in U.S. dollars), each on December 4, 2018, the last trading day before the day on which Mereo and OncoMed announced the execution of the Merger Agreement, and on [ ● ], 2019, the last practicable trading day before the date of this proxy statement/prospectus. This table also shows the equivalent value of the Share Consideration to be received by OncoMed stockholders in the Merger per share of OncoMed common stock, which was calculated by multiplying the closing price of a Mereo Share on AIM as of the dates specified (converted into U.S. dollars at the Federal Reserve Bank of New York’s reported U.S. dollar to pound sterling exchange rate on such dates) by the Implied OncoMed Ownership, assuming no adjustment to the Exchange Ratio for the net cash held by OncoMed at the time of the closing of the Merger.

The market prices of Mereo Shares and shares of OncoMed common stock, and the currency exchange rates, will fluctuate before the OncoMed Special Meeting and before the Merger is consummated. You should obtain current stock or currency rate quotations from a newspaper, the Internet or your broker or banker.

The Implied OncoMed Ownership referenced above is an estimate only and the actual percentage of outstanding equity interests in the Combined Company to be held by former OncoMed stockholders immediately following the closing of the Merger will be determined by the final Exchange Ratio, calculated pursuant to a formula described in more detail in the Merger Agreement and elsewhere in this proxy statement/prospectus. In particular, if OncoMed’s net cash at the time of the closing of the Merger is less than $36.5 million, the Exchange Ratio will be reduced by an amount that is greater than the amount that would otherwise reflect the difference between the target closing net cash and actual closing net cash on a dollar-for-dollar basis. Because the number of Mereo ADSs to be exchanged for each share of OncoMed common stock will be adjusted based on the net cash held by OncoMed at the time of the closing of the Merger and will be unaffected by any increase or decrease in exchange rates or in the share price of Mereo Shares between now and the closing of the Merger, the notional value of the Merger Consideration and the exact number of Mereo ADSs that will be issued to OncoMed stockholders as of the date of the OncoMed Special Meeting and as of the date of the closing of the Merger cannot be determined with precision in advance of the Effective Time.

Mereo’s Dividend Policy .    Mereo has never paid or declared any cash dividends on its ordinary shares, and does not anticipate paying any cash dividends on its ordinary shares in the foreseeable future. Mereo intends to retain all available funds and any future earnings to fund the development and expansion of its business. Under English law, among other things, Mereo may only pay dividends if it has sufficient distributable reserves (on a non-consolidated basis), which are calculated as Mereo’s accumulated realized profits that have not been previously distributed or capitalized less its

accumulated realized losses, so far as such losses have not been previously written off in a reduction or reorganization of capital.

In addition, the terms of Mereo’s existing loan agreement with Silicon Valley Bank and Kreos Capital V (UK) Limited (“Kreos”), preclude Mereo from paying cash dividends without Kreos’s consent.

OncoMed’s Dividend Policy .    OncoMed has never declared or paid cash dividends on its capital stock. OncoMed intends to retain all available funds and any future earnings to fund the development and expansion of its business and does not anticipate paying any cash dividends in the foreseeable future.

Risk Factors (page 41)

You should carefully read this proxy statement/prospectus and especially consider the factors discussed in “Risk Factors” in connection with your consideration of the Merger before deciding whether to vote for approval of the Merger Agreement and the Merger.

The OncoMed Special Meeting (page 105)

Date, Time and Place of the OncoMed Special Meeting

The OncoMed Special Meeting is scheduled to be held at OncoMed’s headquarters located at 800 Chesapeake Drive, Redwood City, California 94063, on [ ● ], 2019, at [ ● ], local time.

Purpose

At the OncoMed Special Meeting, OncoMed stockholders will be asked to approve the Merger Proposal, the Advisory Vote Proposal and the Adjournment Proposal.

The OncoMed Board unanimously recommends a vote “FOR” the Merger Proposal, “FOR” the Advisory Vote Proposal and “FOR” the Adjournment Proposal.

Who Can Vote at the OncoMed Special Meeting

Only OncoMed stockholders of record at the close of business on [ ● ], 2019, the record date for the OncoMed Special Meeting, and other persons holding valid proxies for the special meeting will be entitled to attend the OncoMed Special Meeting. As of the record date, there were [ ● ] shares of OncoMed common stock, par value $0.001 per share, issued and outstanding. Each share of common stock is entitled to one vote on each matter properly brought before the OncoMed Special Meeting.

As of the close of business on the record date, approximately [ ● ]% of the outstanding shares of OncoMed common stock were held by OncoMed’s directors and executive officers and their affiliates, including Delphi Ventures and The Vertical Group. In accordance with the support agreements, such OncoMed directors and executive officers granted an irrevocable proxy to Mereo to vote such individual’s shares in favor of the adoption of the Merger Agreement, and against any alternative proposal and against any action or agreement that would frustrate the purposes, or prevent, delay or otherwise adversely affect the consummation, of the transactions contemplated by the Merger Agreement.

Voting Procedures

Record holders of shares of OncoMed common stock may submit proxies by completing, signing and dating their proxy cards for the OncoMed Special Meeting and mailing them in the accompanying pre-addressed envelopes. OncoMed stockholders who hold shares in “street name” may vote by mail

by completing, signing and dating the voting instruction cards for the OncoMed Special Meeting provided by their brokers or other nominees and mailing them in the accompanying pre-addressed envelopes. Proxies and voting instruction forms submitted by mail must be received no later than [ ● ], 2019 at 11:59 p.m. Eastern Time to be voted at the OncoMed Special Meeting. OncoMed stockholders may also submit proxies over the Internet at the web address shown on the proxy card or by calling the telephone number shown on the proxy card. The Internet and telephone voting facilities will close at 11:59 p.m., Eastern Time, on [ ● ], 2019. The availability of Internet and telephone voting for shares held in “street name” will depend on the voting processes of your broker or other nominee.

Share Ownership and Voting by OncoMed Directors and Executive Officers (page 107)

At the close of business on the record date for the OncoMed Special Meeting, directors and executive officers of OncoMed (together with certain of their respective affiliates, including Delphi Ventures and The Vertical Group) beneficially owned and were entitled to vote approximately [ ● ]% of the shares of OncoMed common stock outstanding on that date. Simultaneously with the execution and delivery of the Merger Agreement, each of the directors and executive officers of OncoMed, in their respective capacities as stockholders of OncoMed (together with certain of their respective affiliates, including Delphi Ventures and The Vertical Group), entered into support agreements with Mereo pursuant to which such individuals granted an irrevocable proxy to Mereo, among other things, to vote their respective shares of OncoMed common stock in favor of the adoption of the Merger Agreement.

Recommendation of the OncoMed Board and its Reasons for the Merger (pages 105 and 126)

After careful consideration, at a meeting of the OncoMed Board held on December 4, 2018, the OncoMed Board unanimously determined that the Merger Agreement and the transactions contemplated by the Merger Agreement are advisable and in the best interests of OncoMed stockholders and unanimously approved the Merger Agreement.

The OncoMed Board unanimously recommends that OncoMed stockholders vote “FOR” the Merger Proposal, “FOR” the Advisory Vote Proposal and “FOR” the Adjournment Proposal.

Opinion of OncoMed’s Financial Advisor (page 129 and Annex C)

OncoMed retained Leerink Partners LLC (now known as SVB Leerink LLC, and referred to in this proxy statement/prospectus as “Leerink Partners”) as its financial advisor in connection with this transaction. The OncoMed Board selected Leerink Partners to act as OncoMed’s financial advisor based on Leerink Partners’ qualifications, reputation, experience and expertise in the biopharmaceuticals industry, its knowledge of and involvement in recent transactions in the biopharmaceutical industry, and its longstanding relationship and familiarity with OncoMed and its business. Leerink Partners is an internationally recognized investment banking firm that has substantial experience in transactions similar to this transaction. In connection with this engagement, OncoMed requested that Leerink Partners evaluate the fairness, from a financial point of view, to the holders of the outstanding shares of OncoMed common stock (other than (i) shares held as treasury stock immediately prior to the effective time of the Merger and (ii) shares that are outstanding immediately prior to the effective time of the Merger and which are held by stockholders who have exercised and perfected appraisal rights for such shares in accordance with the General Corporation Law of the State of Delaware (the “DGCL”) (collectively, the “Excluded Shares”)) of the Merger Consideration proposed to be paid to such holders pursuant to the Merger Agreement. On December 4, 2018, Leerink Partners rendered to the OncoMed Board its oral opinion, which was subsequently confirmed by delivery of a written opinion dated December 4, 2018, that, as of such date and based upon and subject to the assumptions made and limitations upon the review undertaken by Leerink Partners in preparing its opinion, the merger consideration to be paid to the holders of the outstanding shares of OncoMed common stock (other than Excluded Shares) pursuant to the terms of the Merger Agreement was fair, from a financial point of view, to such holders.

Leerink Partners’ financial advisory services and opinion were provided for the information and assistance of the members of the OncoMed Board (in their capacity as directors and not in any other capacity) in connection with and for purposes of the OncoMed Board’s consideration of this transaction and the Leerink Partners opinion addressed only the fairness, from a financial point of view, as of the date thereof, to the holders of the outstanding shares of OncoMed common stock (other than Excluded Shares) of the Merger Consideration to be paid to such holders pursuant to the terms of the Merger Agreement. The Leerink Partners opinion did not address any other term or aspect of the Merger Agreement or this transaction and does not constitute a recommendation to any stockholder of OncoMed as to whether or how such holder should vote with respect to the Merger or otherwise act with respect to this transaction or any other matter. Leerink Partners has provided its written consent to the reproduction of its opinion in this proxy statement/prospectus.

The full text of the Leerink Partners written opinion, dated December 4, 2018, which describes the assumptions made and limitations upon the review undertaken by Leerink Partners in preparing its opinion, is attached hereto as Annex C and is incorporated by reference herein. You should read the opinion carefully in its entirety.

Accounting Treatment (page 138)

The merger will be accounted for in accordance with International Financial Reporting Standards (“IFRS”) as issued by the International Accounting Standards Board (“IASB”), and in particular, with IFRS 3, Business Combinations (“IFRS 3”), under which the Merger qualifies as the acquisition of OncoMed by Mereo. On the date of the acquisition, the identifiable assets and liabilities of OncoMed will be recorded by Mereo at their respective fair values. Any excess of the purchase price over the net fair value at date of the acquisition of the identifiable assets acquired and liabilities assumed will be recognized as goodwill. Should the fair value of the assets and liabilities at the date of acquisition exceed the value of the consideration, a gain will be recognized on acquisition in accordance with IFRS 3 and recorded in the statement of operations of the Combined Company.

Interests of OncoMed’s Directors and Executive Officers in the Merger (page 138)

In considering the recommendation of the OncoMed Board to adopt the Merger Agreement and approve the transactions contemplated by the Merger Agreement, OncoMed stockholders should be aware that some of the OncoMed directors and executive officers have interests in the merger and have arrangements that are different from, or in addition to, those of OncoMed stockholders generally, including, but not limited to, the following:

These interests and arrangements may create potential conflicts of interest. The OncoMed Board was aware of these potential conflicts of interest and considered them, among other matters, in reaching its decision to approve the Merger Agreement and the transactions contemplated by the Merger Agreement.

Board of Directors and Senior Management of the Combined Company (page 138)

Immediately following the Effective Time, two members of the existing OncoMed Board, Michael Wyzga and Dr. Deepika Pakianathan, will be appointed to the Mereo Board. Dr. Denise Scots-Knight will continue as Chief Executive Officer of the Combined Company and Richard Jones will continue as the Chief Financial Officer of the Combined Company. Dr. Peter Fellner will continue in his role as Chairman of the Mereo Board.

Mereo’s Reasons for the Merger (page 142)

At its meeting on December 5, 2018, the Mereo Board unanimously (1) determined that the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement are advisable and are fair to and in the best interests of Mereo and its shareholders as a whole, (2) approved the Merger Agreement, the Merger and the other transactions contemplated by the Merger Agreement, and (3) resolved to recommend to Mereo’s shareholders that they should approve the transactions contemplated by the Merger Agreement, should such approval be required.

The Mereo Board believes (1) that the combination of Mereo’s biopharmaceutical portfolio of four assets with OncoMed’s two lead assets will create a diversified combined portfolio, resulting in an increased number of potential near-term catalysts with a core focus remaining on Mereo’s strategy to target rare diseases, (2) that the cash position of the Combined Company will provide an extended operational runway, with the potential for such runway to be extended significantly through partnering deals, and (3) that a Nasdaq listing, in addition to Mereo’s existing AIM listing, will provide a diversified international shareholder base for the Combined Company. The Mereo Board considered a variety of other factors in favor of the Merger, which are discussed further in the section entitled “The Merger—Mereo’s Reasons for the Merger” located elsewhere in this proxy statement/prospectus.

Appraisal Rights (page 144 and Annex D)

Record holders of OncoMed common stock who do not vote in favor of the Merger Proposal and otherwise comply with the requirements and procedures of Section 262 of the DGCL are entitled to exercise appraisal rights, which generally entitle stockholders to receive in lieu of the Merger Consideration a cash payment of an amount determined by the Court of Chancery of the State of Delaware (the “Court of Chancery”) to be equal to the fair value of their OncoMed common stock as of the Effective Time. The fair value of OncoMed common stock as of the Effective Time could be less than, more than or the same as the Merger Consideration. Stockholders will not know the appraised fair value at the time such holders must elect whether to seek appraisal.

To seek appraisal, you must deliver a written demand for appraisal to OncoMed before the vote on the adoption of the Merger Agreement at the OncoMed Special Meeting, and you must not vote in favor of the adoption of the Merger Agreement. Failure to follow exactly the procedures specified under the DGCL will result in the loss of appraisal rights.

A summary description of the appraisal rights available to holders of OncoMed common stock under the DGCL and the procedures required to exercise statutory appraisal rights is included in “The Merger—Appraisal Rights.” The full text of Section 262 of the DGCL is attached as Annex D to this proxy statement/prospectus.

Due to the complexity of the procedures described above, OncoMed stockholders who are considering exercising such rights are encouraged to seek the advice of legal counsel.

Listing of the Mereo ADSs and Mereo Shares (page 148)

The approval for listing of the Mereo ADSs on Nasdaq and the confirmation by AIM that it will admit the Mereo Shares underlying the Mereo ADSs to trading, in each case subject only to official notice of issuance, are each a condition to the obligations of Mereo and OncoMed to consummate the Merger. Mereo intends to apply to list the Mereo ADSs on Nasdaq and to list the Mereo Shares underlying the Mereo ADSs on AIM. Mereo expects that the Mereo ADSs will trade on Nasdaq under the symbol “MREO.”

Delisting and Deregistration of OncoMed Common Stock (page 149)

If the Merger is completed, OncoMed’s common stock will be deregistered under the Exchange Act and will cease to be listed for trading on Nasdaq.

Litigation Related to the Merger (page 149)

It is a condition to the Merger that no temporary restraining order, preliminary or permanent injunction or other order preventing the consummation of the Merger Agreement or the transactions contemplated thereby shall have been issued by any court of competent jurisdiction or other governmental authority of competent jurisdiction and remain in effect. Neither OncoMed nor Mereo is aware of any lawsuit or proceeding specific to the Merger having been filed to date. If such a lawsuit or other proceeding is commenced and if in any such litigation or proceeding a plaintiff is successful in obtaining a restraining order or injunction prohibiting the consummation of the Merger Agreement or the transactions contemplated thereby, then the closing of the Merger may be delayed or may never occur. Even if the Merger is permitted to occur, the parties may be required to pay damages, fees or expenses in respect of claims related to the Merger or the transactions contemplated thereby.

The Merger Agreement (page 150 and Annex A)

A copy of the Agreement and Plan of Merger and Reorganization is attached as Annex A to this proxy statement/prospectus. You should read the entire Merger Agreement carefully because it is the principal document governing the Merger. For a further discussion of the Merger Agreement, see the section entitled “The Merger Agreement” located elsewhere in this proxy statement/prospectus.

No Solicitation of Offers (page 162)

As more fully described in this proxy statement/prospectus and in the Merger Agreement, and subject to the exceptions described below and in the Merger Agreement, each of OncoMed and Mereo has agreed, among other things, that it will not, directly or indirectly:

Each party has further agreed (1) subject to any regulatory obligations of such party under applicable law, to promptly advise the other party orally and in writing upon receipt of any acquisition proposal or acquisition inquiry, and (2) to cease any discussions, negotiations or communications with any person with respect to any acquisition proposal as of the date of the Merger Agreement.

However, at any time prior to the approval and adoption of the Merger Agreement by OncoMed stockholders, in the case of OncoMed, or Mereo shareholders, in the case of Mereo, each party may furnish non-public information regarding such party and its subsidiaries to, and enter into discussions or negotiations with, any person in response to a bona fide written acquisition proposal which such party’s board of directors determines in good faith, after consultation with such party’s financial advisors and outside legal counsel, constitutes or is reasonably likely to result in a superior offer, provided that:

In addition, (1) Mereo’s obligation not to solicit offers shall not require Mereo to take any action, or prevent Mereo from taking any action, which Mereo reasonably determines, having consulted with its outside legal counsel, would be inconsistent with or in breach of the U.K. City Code on Takeovers and Mergers (the “U.K. City Code”), and (2) OncoMed’s obligation not to solicit offers shall not require OncoMed to take any action, or prevent OncoMed from taking any action, which OncoMed reasonably determines, having consulted with its outside legal counsel, would be inconsistent with or in breach of OncoMed’s obligations under the DGCL.

Change of Recommendation (page 163)

OncoMed’s Board Recommendation

Subject to the exceptions described below and in the Merger Agreement, OncoMed has agreed:

Notwithstanding the above, the OncoMed Board may withhold, amend, withdraw or modify the OncoMed Board Recommendation in a manner adverse to Mereo (an “OncoMed Adverse Recommendation Change”) (so long as OncoMed has provided prior written notice to Mereo of the OncoMed Board’s intention to make an OncoMed Adverse Recommendation Change at least four business days in advance of taking such action (the “Notice Period”)) if, and only if, following receipt of a superior offer:

Mereo’s Board Recommendation

Subject to the exceptions described below and in the Merger Agreement, Mereo has agreed:

Notwithstanding the above, the Mereo Board may withhold, amend, withdraw or modify the Mereo Board Recommendation in a manner adverse to OncoMed (a “Mereo Adverse Recommendation Change”) (so long as Mereo has provided prior written notice to OncoMed of the Mereo Board’s intention to make a Mereo Adverse Recommendation Change at least four business days in advance of taking such action) if, and only if, following receipt of a superior offer:

Indemnification and Insurance (page 165)

Pursuant to the terms of the Merger Agreement, OncoMed’s directors and executive officers will be entitled to certain ongoing indemnification and coverage under directors’ and officers’ liability insurance policies of, and the organizational documents of, OncoMed and Mereo. See the section entitled “The Merger Agreement—Indemnification and Insurance” located elsewhere in this proxy statement/prospectus.

Conditions to Closing (page 167)

Each party’s obligation to effect the Merger is subject to satisfaction or, to the extent permitted by applicable law, mutual written waiver by each of the parties of the following conditions:

The obligations of Mereo, HoldCo and Merger Sub to effect the Merger is further subject to the satisfaction or waiver of the following conditions:

OncoMed’s obligation to effect the Merger is further subject to the satisfaction or waiver of the following conditions:

Termination Events (page 169)

The Merger Agreement may be terminated at any time prior to the Effective Time by mutual written consent of Mereo and OncoMed, and either party may terminate the Merger Agreement in the following circumstances:

OncoMed may terminate the Merger Agreement at any time prior to the Effective Time as follows:

Mereo may terminate the Merger Agreement at any time prior to the Effective Time as follows:

Termination Fees (page 171)

OncoMed will be required to pay to Mereo a termination fee of $1,721,193 (subject to any adjustments for VAT) in cash if the Merger Agreement is terminated by Mereo in the event that:

In addition to the payment of the termination fee, OncoMed shall reimburse Mereo for all reasonable out-of-pocket fees and expenses incurred by Mereo in connection with the Merger Agreement and the transactions contemplated by the Merger Agreement up to a maximum of $750,000 (subject to any adjustments for VAT) if the Merger Agreement is terminated by Mereo in the event that:

Mereo will be required to pay to OncoMed a termination fee of $1,721,193 (subject to any adjustments for VAT) in cash if the Merger Agreement is terminated by Mereo or OncoMed in the event that:

Mereo will be required to pay to OncoMed a termination fee of $1,721,193 (subject to any adjustments for VAT) in cash if the Merger Agreement is terminated by OncoMed in the event that:

In addition to the payment of the termination fee, Mereo shall reimburse OncoMed for all reasonable out-of-pocket fees and expenses incurred by OncoMed in connection with the Merger Agreement and the transactions contemplated by the Merger Agreement up to a maximum of $750,000 (subject to any adjustments for VAT) if the Merger Agreement is terminated by OncoMed in the event that:

Material U.S. Federal Income Tax Considerations (page 283)

The exchange of OncoMed common stock for merger consideration in the Merger is expected to be a taxable transaction for U.S. federal income tax purposes to U.S. Holders (as defined in “Material U.S. Federal Income Tax Considerations”). Please carefully review the information under “Material U.S. Federal Income Tax Considerations” beginning on page 283 of this proxy statement/prospectus for a description of material U.S. federal income tax consequences of the Merger to U.S. Holders. The tax consequences to you will depend on your own situation. You are urged to consult your tax advisors as to the specific tax consequences to you of the Merger and your receipt of the Merger Consideration, including the applicability and effect of U.S. federal, state, local and non-U.S. income and other tax laws in light of your particular circumstances.

Material U.K. Tax Considerations (page 290)

For a summary of the anticipated material U.K. tax considerations of ownership of Mereo ADSs, please see the section of this proxy statement/prospectus entitled “Material U.K. Tax Considerations.”

Comparison of Shareholder Rights (page 315)

As a result of the Merger, OncoMed stockholders will become holders of Mereo ADSs, and will have different rights as holders of Mereo ADSs than they had as holders of OncoMed common stock. The differences between the rights of these respective holders result from the differences among (1) English and Delaware law, (2) the respective governing documents of Mereo and OncoMed, and (3) the terms of the deposit agreement among Citibank, Mereo and the holders and beneficial owners of Mereo ADSs. For additional information, see “Comparison of Shareholder Rights” and “Description of the Mereo ADSs.” For a copy of OncoMed’s current certificate of incorporation or bylaws, see “Where You Can Find More Information.” Mereo’s articles of association as of the date hereof are included as an exhibit to the registration statement of which this proxy statement/prospectus is a part.