At KOAK, how can I get a passenger from the GA terminal to the commercial terminal?
This is a specific question but I think my advice applies to most FBOs.
I fly out of OAK and your best bet is to try signature. Buy some fuel, ask nicely and if they aren't too busy they might be able to accommodate you. 
They have given me rides on the ramp before when my usual gate was closed due to a power outage.

If not, then call a rideshare/cab