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It would be V.O. unless the dialogue is someone actually standing in a darkened room (and at some point the audience would be clued into this fact by introducing light). In that one special case, it would be O.S.
O.S. or "off-screen" is dialogue from a character physically present in the scene, just not visible through the camera. TV scripts sometimes use O.C. (off-camera) instead.
V.O. or "voice over" is dialogue that the audience hears but someone physically in the scene could not, such as narration or someone's thoughts.
Related is M.O.S. (opinions differ on what the letters stand for) which is dialogue that someone in the scene could hear but the audience cannot, such as whispering or background conversation. The screenplay typically doesn't specify the words spoken.
The story goes that a German-born director (perhaps Josef von Sternberg, who discovered Marlene Dietrich) wanted to shoot a scene without sound and told the crew to shoot "mit out sound," a phrase which the crew found humorous and thus proliferated it. It is most commonly used to show impending impact of some kind.
Thanks for the clarification, Frank. Much appreciated.
What does M.O.S. stand for, exactly?
The more credible possibilities are collected on the Wikipedia page https://en.wikipedia.org/wiki/MOS_(filmmaking), and I would lean toward the first one discussed: "motor only shot" derived from the instructions given to the sound crew when they needed to have their equipment's motors running to keep in sync with the video camera but no need to actually record the sounds.
Others believe it is an in-joke aimed at German immigrants from around the time of the first talkies, who might pronounce "without sound" as "mit out sound." Given that the US film industry is centered in California, the snowflakiest part of the country, I'm pretty sure a term that made fun of a minority would have been replaced by now.