Exhibit 5.1

Tel +713.758.2222  Fax +713.758.2346

September 11, 2017

Oasis Midstream Partners LP

1001 Fannin St., Suite 1500

Houston, Texas 77002

Ladies and Gentlemen

We have acted as special counsel to Oasis Midstream Partners LP, a Delaware limited partnership (the “Partnership”), in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of the offering and sale by the Partnership of up to an aggregate of 7,500,000 common units representing limited partner interests in the Partnership (the “Common Units”) and up to an additional Common 1,125,000 Units pursuant to the underwriters’ option to purchase additional Common Units.

We are rendering this opinion as of the time the Partnership’s Registration Statement on Form S-1 (File No. 333-217976), as amended (the “Registration Statement”), to which this opinion is an exhibit and relating to the Common Units, becomes effective in accordance with Section 8(a) of the Securities Act. The term “Common Units” shall include any additional common units representing limited partner interests in the Partnership registered pursuant to Rule 462(b) under the Securities Act in connection with the offering contemplated by the Registration Statement.

As the basis for the opinion hereinafter expressed, we examined such statutes, including the Delaware Revised Uniform Limited Partnership Act (the “Delaware LP Act”), and the Partnership’s respective records and documents, certificates of the Partnership and public officials, and other instruments and documents as we deemed necessary or advisable for the purposes of this opinion. In such examination, we have assumed (i) the authenticity of all documents submitted to us as originals and the conformity with the original documents of all documents submitted to us as copies and (ii) that a definitive underwriting agreement in the form filed as an exhibit to the Registration Statement with respect to the sale of the Common Units will have been duly authorized and validly executed and delivered by the Partnership and the other parties thereto.

Based on the foregoing and on such legal considerations as we deem relevant, we are of the opinion that (i) the Partnership has been duly formed and is validly existing as a limited partnership under the Delaware LP Act, (ii) the Common Units, when issued and delivered on behalf of the Partnership against payment therefor as described in the Registration Statement, will be duly authorized, validly issued, fully paid and non-assessable and (iii) purchasers of the Common Units will have no obligation under the Delaware LP Act, the Partnership’s governing documents or any resolution or other action taken under the Partnership’s governing documents, to make further payments to the Partnership or its creditors for their purchase of Common Units or contributions to the Partnership or its creditors solely by reason of their ownership of Common Units or their status as limited partners of the Partnership and no personal liability for the debts, obligations and liabilities of the Partnership, whether arising in contract, tort or otherwise, solely by reason of being limited partners of the Partnership.

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

September 11, 2017 Page 2

The foregoing opinion is limited to the federal laws of the United States of America, the Constitution of the State of Delaware and the Delaware LP Act, each as interpreted by the federal courts and the courts of the State of Delaware, and we are expressing no opinion as to the effect of the laws of any other jurisdiction.

We hereby consent to the reference to us under the heading “Validity of Our Common Units” in the Registration Statement and the filing of this opinion as an exhibit to the Registration Statement. We further consent to the incorporation by reference of this letter and consent into any registration statement filed pursuant to Rule 462(b) under the Securities Act with respect to the Common Units. By 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 and the rules and regulations thereunder.

Very truly yours,

/s/ Vinson & Elkins L.L.P.