you're right, but you're wrong!
it
would be V.O. [or VO] since the character speaking is not physically anywhere in the scene and is only heard... but it's not typed V/O in standard screenwriting format...
O.S. [or OS] is used when the character speaking is
in the scene, but just
not seen at the time s/he's speaking... such as when someone is speaking from an adjoining room or behind a closed door...
V.O. is also used for all 'over a speaker' type dialog... such as for the other party to a telephone conversation, airport announcements, and such...
fyi, for tv sitcoms, O.C. [off camera] is used instead of O.S. [off screen]...
hope that helps... love and hugs, maia