The shorthand way of distinguishing them is that the Department of Justice gives guidance to federal agencies and establishes employer policy and has oversight of all the agencies for compliance.
What the new office in the National Archives does is something that is new and distinct, and that's offering mediation services to resolve disputes. The idea there is that as a non-exclusive alternative to litigation, a requester could go to OGIS's new office and go through a mediation process if they have a dispute with an agency.