When you have a character talking on the phone who is not in the scene that the audience is watching (e.g. Bill is in a phone booth talking to Jim who we only hear but never see) — do you use (O.S.) or (O.C.) or something else?
I would use O.S., which means “off screen.” I think the distinction is supposed to be that O.C. (“off camera”) applies when the speaker is physically in the same space as the person he’s talking to, but just not on camera, while O.S. is when speaker and listener are in different places.
Your case is definitely the latter. It would look like this:
Bill holds up a one-sec finger while he answers the payphone.
Where the hell are you? He’s waking up, and I’m out of demerol.
I don’t think the distinction between the two terms is all that useful. In fact, I never use O.C., even in situations where it would probably apply — I just use O.S., and no one is ever confused.
I can’t say for certain what my aversion to O.C. is. It may be that on a subconscious level, I know that the “C.” stands for “camera,” and I try to never refer to the camera itself. I think it takes the reader out of the story, reminding them that what they’re reading is just a script.
However, there’s absolutely nothing wrong with O.C. — most readers will know what it means. So if it floats your boat, by all means use it in appropriate situations. Which does not include this one.