Which groups of people would work according to an operational level agreement?

Business units are party to Service Level Agreements (SLAs) with the IT organization, not OLAs, which exist solely between internal IT groups.

OLAs are not applicable to all stakeholders - external parties and business units are governed by SLAs and Underpinning Contracts, not OLAs.

External IT teams or third-party suppliers are bound by Underpinning Contracts (UCs), not OLAs, which are strictly internal documents.

OLAs are agreements between internal IT teams or departments within the same service provider organization, defining how they will cooperate to meet the commitments outlined in Service Level Agreements. They are entirely internal and do not extend to business units or external parties.