Oss (sometimes spelled "osu" or "ossu") is borrowed from Japanese budo and karate, where it carries connotations of perseverance and respect. In BJJ academies it has become a catch-all acknowledgement: coaches say it when they want the class to confirm they understand ("make sense? — oss"), students say it when they tap, pair up, or respond to instructions.
The term is more common in traditionally structured academies and less common in modern sport-BJJ or wrestling-heavy gyms where the language is more casual. Neither is more correct. New training partners take cues from the room.
Usage: say oss when the coach explains a technique and expects acknowledgement; when you receive a tap from your partner; or when a senior belt greets you at the mat.