OPA draws some inspiration from XACML. For example, OPA helps systems separate policy decision-making from policy enforcement. Decoupling means that policy decisions can be more easily updated and more readily understood.

XACML as a specification covers multiple things (e.g., access control language, overall architecture, etc.) If you wanted to map OPA to a component in XACML, it's closest to the PDP.

That being said, OPA gives you a more expressive language to author policy. Also, OPA is intended to be used as building block in other systems. OPA exposes APIs to offload policy decisions from services and manage the policies and data that are evaluated. One of the non-goals of OPA is management-plane concerns around policy storage, policy administration, etc.