Depends entirely on the agent. They may be co-terminals for booking some fares, but (as is mentioned in the various other threads), the only co-terminals for same-day standby are:

IAD-DCA
JFK-LGA
ORD-MDW


If it turns into an irregular ops day, all bets are off, and UA will probably gladly move you to another flight with the understanding that you'll be responsible to get to your originally-ticketed destination (SFO) once they get you to OAK.