Exhibit 5.1

July 8, 2019

Liberty Oilfield Services Inc.

950 17^th Street, Suite 2400

Denver, Colorado 80202

Ladies and Gentlemen:

We have acted as counsel to Liberty Oilfield Services Inc., a Delaware corporation (the “Company”), with respect to the preparation of the Registration Statement on Form S-3 (the “Registration Statement”) filed on or about the date hereof with the Securities and Exchange Commission (the “Commission”), including a base prospectus (the “Base Prospectus”), which provides that it will be supplemented by one or more prospectus supplements (each such prospectus supplement, together with the Base Prospectus, a “Prospectus”), under the Securities Act of 1933, as amended (the “Act”), relating to the registration for issue and sale by the Company from time to time of an indeterminate amount of shares of the Company’s Class A common stock, $0.01 par value per share (“Common Stock”).

This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related applicable Prospectus, other than as expressly stated herein with respect to the issue of the Common Stock.

In connection with the opinions expressed herein, we have examined, among other things, (i) the amended and restated certificate of incorporation and the amended and restated bylaws of the Company, (ii) the Registration Statement, (iii) the Prospectus and (iv) the records of corporate proceedings that have occurred prior to the date hereof with respect to the Registration Statement. We have also reviewed such questions of law as we have deemed necessary or appropriate. As to matters of fact relevant to the opinions expressed herein, and as to factual matters arising in connection with our examination of corporate documents, records and other documents and writings, we relied upon certificates and other communications of corporate officers of the Company, without further investigation as to the facts set forth therein.

In connection with rendering the opinions set forth below, we have assumed that (i) all information contained in all documents reviewed by us is true and correct; (ii) all signatures on all documents examined by us are genuine; (iii) all documents submitted to us as originals are authentic and all documents submitted to us as copies conform to the originals of those documents; (iv) the Registration Statement and any subsequent amendments (including post-effective amendments) will be effective and comply with all applicable laws; (v) all Common Stock will be issued and sold in compliance with applicable federal and state securities laws and in the manner specified in the Registration Statement and the applicable prospectus supplement to the Prospectus; (vi) one or more prospectus supplements to the Prospectus will have been prepared and filed with the Commission describing the Common Stock offered thereby; and (vii) a definitive purchase, underwriting or similar agreement with respect to any Common Stock offered will have been duly authorized and validly executed and delivered by the Company and the other parties thereto.

Based on the foregoing, and subject to the assumptions, qualifications, limitations, and exceptions set forth herein, we are of the opinion that:

Vinson & Elkins LLP Attorneys at Law Austin Beijing Dallas Dubai Hong Kong Houston London New York Richmond Riyadh San Francisco Tokyo Washington 1001 Fannin Street, Suite 2500 Houston, TX 77002-6760 Tel +1.713.758.2222   Fax +1.713.758.2346   www.velaw.com

July 8, 2019 Page 2

When an issuance of Common Stock has been duly authorized by all necessary corporate action of the Company, upon issuance, delivery and payment therefor in an amount not less than the par value thereof in the manner contemplated by the applicable Prospectus and by such corporate action, and in total amounts and numbers of shares that do not exceed the respective total amounts and numbers of shares (a) available under the amended and restated certificate of incorporation, and (b) authorized by the board of directors in connection with the offering contemplated by the applicable Prospectus, such shares of Common Stock will be validly issued, fully paid and nonassessable. In rendering the foregoing opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the General Corporation Law of the State of Delaware.

We express no opinions concerning the enforceability of indemnification provisions to the extent they purport to relate to liabilities resulting from or based upon negligence or any violation of federal or state securities or blue sky laws. We express no opinion as to any matters other than as expressly set forth above, and no opinion on any other matter may be inferred or implied herefrom.

The foregoing opinions are limited in all respects to the Delaware General Corporation Law (including the applicable provisions of the Delaware Constitution and the reported judicial decisions interpreting these laws) and the federal laws of the United States of America, and we are expressing no opinion as to the effect of the laws of any other jurisdiction, domestic or foreign.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the use of our name in the Prospectuses forming a part of the Registration Statement under the captions “Legal Matters.” 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.

Very truly yours,

/s/ Vinson & Elkins L.L.P.