According to my understanding there are two processes in resources management knowledge area ( 9.3 Acquire Resources & 9.4 Develop Reference PMBOK 6th Ed ) these two processes laying at Execution Group and resulted an updated of OPA.
Also the process of 4.4 Manage Project Knowledge ( in integration management knowledge area) which laying at (Execution group) resulted an updated of OPA.
In summary there are about ( 4 - 5 ) processes can update the OPA including closing project or phase.
BR,
Mansour Saving Changes...
RAJESH K LProject Manager, PMP| Bharat Electronics, Bengaluru, IndiaBengaluru, Karnataka, India