Having thought about this a little, I wonder if Gottlieb used the letter O for a relay to operate the outhole solenoid, in conjunction with a switch on the motor. An e/m Gottlieb single player relies on a switch fitted to the track between outhole and ball shooter to 'step' the Ball Count Unit (usually in the backbox). The unit flies back to its Zero position during the reset procedure, then steps to Ball 1 as the ball passes over the track switch - until then the flippers and most other playfield solenoids don't work. So if the ball remains in the outhole the start sequence stalls. Is that where you are (or were)?
It's been a while since I owned a Gottlieb 'wedge-head', but I'd suppose that a ball closing the outhole switch will, once the reset sequence is ready for it, operate the outhole relay, and then the motor will turn to actually kick the outhole solenoid. The ball returning to the outhole then repeats this until the final ball, which by-passes the solenoid operation through some contacts on the 'disc' of the Ball Count Unit.
If you have the schematic diagram, look at the circuit for O relay, and try to relate the pathways and contacts to the actual machine (I appreciate this isn't as easy as it sounds). One circuit can track right through the whole machine. If I'm on the right trail here, this one will be on the playfield (outhole switch), motor panel (relay & Motor) and backbox (Ball Count Unit).
