TV uses O.S. too. I think there's a slight difference (again, it's "I think").
O.S. is generally used. When a character is speaking off somewhere (on a phone, in the bathroom yelling out, etc.)
O.C. seems to me a little more camera specific. That is, if the camera is on a close up of a character, and he's having a conversation with another character (who is present in the same scene but not in the close up), then the other character would be O.C.
That's what I think...