...I am told that OSHA requires all employers to ensure all electrical equipment is NRTL listed.
Yes and no.
OSHA section 1910.303(b)(1)(i) requires, in general, that electrical equipment be suitable for installation and use in conformity with the provisions of that and the associated parts of OSHA. Generally, this provision is parallel to (and historically originated with) the essentially identical requirement in the National Electrical Code (NEC), an advisory consensus standard published by the National Fire Protection Association. Both OSHA and the NEC provide that suitability of equipment for an identified purpose may be evidenced by listing or labeling for that identified purpose.
As an alternative to listing or labeling, it's possible in theory to convince an OSHA enforcement authority, local electrical inspector enforcing the locally-adopted-by-reference NEC, or buying-organization purchasing agent, technician or engineer "following the rules" that directly provided documentation of testing and analysis--essentially, a Tech File--shows that an item of electrical equipment is suitable for the prospective user's intended purpose.
In practice this usually is not practical, because individual inspectors do not have the training and time to evaluate such an extensive NRTL-investigation-equivalent analysis, and those inspectors are permitted and expected by custom to reject equipment that is not
prima facie acceptable.
In that regard, an NRTL listing or labeling indication is partly analogous to CE Marking, in that even though it's not legally mandatory it's nonetheless broadly expected because it efficiently communicates to individuals in the purchase-and-installation chain that an item will be safe and effective if used as directed.