OOO, out of office, is the abbreviation for being away from work: the auto-reply that promises a response on return, and in speech, the state itself: “she’s OOO until Monday”. It covers leave, travel and conferences alike; the message is simply “not here, not responding at normal speed”.
Good OOO practice is logistical kindness: dates, a cover contact and honesty about response times. In coordinated teams, OOO status flows from the leave system to calendars and workplace views automatically, so the Thursday plan already knows who is away.