Exhibit 5.1

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Custom Truck One Source, Inc.

7701 Independence Ave

Kansas City, Missouri 64125

Re: Registration Statement on Form S-3; 41,416,667 shares of common stock offered by Custom Truck One Source, Inc., par value $0.0001 per share

Ladies and Gentlemen:

We have acted as special counsel to Custom Truck One Source, Inc., a Delaware corporation (the “Company”), in connection with (A) the resale from time to time by certain selling securityholders (the “Selling Securityholders”) named in the Prospectus (as defined below) of 28,000,000 shares (the “Selling Holder Shares”) of the Company’s common stock, $0.0001 par value per share (the “Common Stock”) and (B) the proposed issuance by the Company of up to 13,416,667 shares of Common Stock (the “Warrant Shares”) issuable upon the exercise of issued and outstanding warrants (the “Warrants”). The Selling Holder Shares and the Warrant Shares are included in a registration statement on Form S-3 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Act”), and the related prospectus included in the Registration Statement (the “Prospectus”), filed with the Securities and Exchange Commission (the “Commission”) on May 6, 2021.

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 Prospectus, other than as expressly stated herein with respect to the issue of the Warrant Shares and the resale of the Selling Holder Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining herein as to the General Corporation Law of the State of Delaware (the “DGCL”), and we express no opinion with respect to the applicability thereto, or the effect thereon, of the laws of any other jurisdiction, or in the case of Delaware, any other laws, or as to any matters of municipal law or the laws of any local agencies within any state.

May 6, 2021

Page 2

 

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof:

1. When the Warrant Shares initially issuable upon exercise of the Warrants shall have been duly registered on the books of the transfer agent and the registrar thereof in the name of or on behalf of the Warrant holders, and have been issued by the Company against payment therefor in the circumstances contemplated by the Warrants, such Warrant Shares will have been duly authorized by all necessary corporate action of the Company, and will be validly issued, fully paid and nonassessable.

2. The Selling Holder Shares have been duly authorized by all necessary corporate action of the Company, and the Selling Holder Shares are validly issued, fully paid and nonassessable.

In rendering the foregoing opinions, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.

With your consent, we have assumed (a) that the Warrants have been duly authorized, executed and delivered by the parties thereto other than the Company, (b) that such Warrants constitute legally valid and binding obligations of the parties thereto other than the Company, enforceable against each of them in accordance with their respective terms and (c) that the status of the Warrants as legally valid and binding obligations of the parties will not be affected by any (i) breaches of, or defaults under, agreements or instruments, (ii) violations of statutes, rules, regulations or court or governmental orders or (iii) failures to obtain required consents, approvals or authorizations from, or to make required registrations, declarations or filings with, governmental authorities.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm contained in the Prospectus under the heading “Legal Matters.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Latham & Watkins LLP