V.O. (voice over) and O.S. (off-screen) are similar terms, but they have slightly different applications. Both are used to indicate that dialogue is spoken by someone not currently seen on the screen; the difference isn’t where the speaker is not, but where the speaker is.
O.S. is used when the character is in the scene location, but not currently on screen. If Sally walks to the other side of the bedroom and into the walk-in closet, and yells unseen about how she’s out of clean socks, O.S. should be used.
In television, especially multicam sitcoms, it is not uncommon to see O.C. (off-camera) used instead of O.S.
V.O. is used when the speaker is not physically in the scene. This could be because the speaker is on the other end of a telephone line or radio broadcast, the speaker is an unseen narrator, or the speaker is a character’s inner-monologue.
This last example is important to note, as it is somewhat counter-intuitive: if an on-screen character’s thoughts are heard, it is V.O., not O.S.
If there is pre-lap dialogue, you can indicate it with PRE-LAP or V.O. Either is acceptable.