Voice over (VO) is fine if someone is narrating a scene either in a documentary or an actor talking about what's happening, eg in flashbacks. However, if the face/mouth of the person speaking can't be seen (eg their back is to the camera) (OS), (OC) or simply (Off)is used after the characters name in the script as in John (OC) "I don't believe it!" or whatever. It's very important for dubbing purposes (ie into other languages) as it gives the translator "free reign", they're not restricted by the time the actor's mouth is moving, or by fitting dubbed words into the same moment as the original voice (as you have to do with things like names or cognates so the dubbed version looks more natural). Further explanation below in the link, the site has other interesting filmscript info.
So chill a little Timothy, it depends on the asker's context :o)