Originally Posted by
ScepticalOptomist
Don’t have any references for you, however, my understanding is that RADIO is only a relay service to ATC and OAC(C) is the ATC unit, potentially housed in different locations.
Either would work in an emergency, however I believe the OACC number should put you in direct contact with ATC and would be my first choice.
That’s the way I interpret it as well, but would feel a lot more confident if I could point to something in black and white. Still searching.