I wouldn’t personally call a V.O. a subset of O.S. , but I guess you could if you wanted to.
*One deals with narration or a voice disconnected from any visuals.
The other deals with a character in scene that we don’t see.
*There are exceptions of course:
-Voice of God all loud and thundering in a scene might be shown as V.O.
-Sometimes being mechanically reproduced such as an answering machine that we see.
-Great exception from Naviobb, in the scene, but heard reading or thoughts.
-Others that I have no idea about.
Then in some other instances:
Things that are obviously technically voiced over… (Like a robot that speaks in a human voice that isn’t a narration, and obviously doesn’t have its human voice actor on screen.) … are not shown as V.O. either, because when the character of the robot is on screen, it’s treated just like any other character that might also need to be heard speaking from off screen, so O.S. is there to make that distinction.
If you can't see the person speaking on the line, yes, they are O.S., but how often in production are they live and being recorded? I doubt it happens very much.
I'm unsure what your point is, but what difference does it make how often they are live and being recorded? You
are not writing HOW to produce a scene, you are writing THE scene, and in that scene your character is either on screen or your character is off screen delivering your dialogue, OR (Generally) a narrator is voicing over the narration you have written.
But in the end, we all chose the way we feel best suits our script, so what are you gonna do?
-Thanks-