Re: How to write a professionally-formatted screenplay
What Mighty Wanderer said about V.O. is incorrect.
O.S. you write when, yes, your character is off-screen, but he is in the scene; the camera just doesn't see him. You write V.O. if there's like a narrator or something like that. The person talking is not in the actual scene, we just hear the voice over what is playing off at that time.
If your character is in the actual scene, and what he says is not a voice over, then you write just his name and nothing else.