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

Business units are parties to Service Level Agreements (SLAs), which define service expectations between the IT provider and the business, not OLAs.

All stakeholders is too broad a scope; OLAs apply specifically to internal IT groups, not every party involved in service delivery.

External IT teams and third-party suppliers are governed by Underpinning Contracts (UCs), not OLAs, because OLAs are strictly internal agreements.

An Operational Level Agreement (OLA) is an internal agreement between the IT service provider and another part of the same organization, specifically between internal IT teams or departments. OLAs define the responsibilities and performance targets for internal support groups to underpin the commitments made in external SLAs. They exist solely within the IT organization to ensure internal teams coordinate effectively.