Exhibit 5.1

OvaScience, Inc.

215 First Street, Suite 240

Cambridge, MA 02142

Ladies and Gentlemen:

We have acted as legal counsel to OvaScience, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing with the Securities and Exchange Commission (the “Commission”) of a Prospectus Supplement, dated March 4, 2014 (the “Prospectus Supplement”), to a Registration Statement (File No. 333-190939) on Form S-3 (the “Registration Statement”), filed by the Company with the Commission under the Securities Act of 1933, as amended (the “Securities Act”). The Prospectus Supplement relates to the sale of up to 5,750,000 shares (the “Shares”) of the Company’s common stock, $0.001 par value per share (the “Common Stock”) to Leerink Partners LLC, as representative of the underwriters (the “Underwriters”), including 750,000 Shares subject to the Underwriters’ over-allotment option, pursuant to an Underwriting Agreement dated March 4, 2014 between the Company and the Underwriters (the “Underwriting Agreement”). The Underwriting Agreement will be filed as an exhibit to a Current Report on Form 8-K and incorporated by reference into the Registration Statement. This opinion is being rendered in connection with the filing of the Prospectus Supplement with the Commission. All capitalized terms used herein and not otherwise defined shall have the respective meanings given to them in the Registration Statement.

In connection with this opinion, we have examined the Company’s Restated Certificate of Incorporation, as amended, and Amended and Restated Bylaws, both as currently in effect; the minutes of all pertinent meetings of the directors of the Company relating to the Registration Statement, the Prospectus Supplement, the Underwriting Agreement and the transactions contemplated thereby; such other records of the corporate proceedings of the Company and certificates of the Company’s officers as we have deemed relevant; the Registration Statement and the exhibits thereto; the Prospectus Supplement; and the Underwriting Agreement.

In our examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified or photostatic copies and the authenticity of the originals of such copies.

Our opinion is limited to the General Corporation Law of the State of Delaware and we express no opinion with respect to the laws of any other jurisdiction. No opinion is expressed herein with respect to the qualification of the Shares under the securities or blue sky laws of any state or any foreign jurisdiction.

Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters. This opinion is based upon currently existing statutes, rules, regulations and judicial decisions, and we disclaim any obligation to advise you of any change in any of

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

BOSTON | LONDON | LOS ANGELES | NEW YORK | SAN DIEGO | SAN FRANCISCO | STAMFORD | WASHINGTON

these sources of law or subsequent legal or factual developments which might affect any matters or opinions set forth herein.

Based upon the foregoing, we are of the opinion that Shares, when issued and sold in accordance with the Underwriting Agreement and the Prospectus Supplement, will be validly issued, fully paid and non-assessable.

We understand that you wish to file this opinion with the Commission as an exhibit to a Current Report on Form 8-K and the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K promulgated under the Securities Act and to reference the firm’s name under the caption “Legal Matters” in the Prospectus Supplement, and we hereby consent thereto. In giving this consent, we do not admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

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