Open interest is the number of option contracts outstanding at a strike and expiration, counted once per long-short pair. Together with daily volume and quote width it forms the liquidity screen that decides whether a theoretical trade is tradeable.
High open interest correlates with tight quotes and orderly exits; thin open interest means any exit negotiates with a market maker holding all the leverage. Liquidity floors, minimum OI and volume on the short strikes plus a quote-width ceiling, are enforced per wing on multi-leg structures, judged by the weakest leg, since closing a spread requires trading all of it. OI updates once daily, so intraday volume carries the fresher signal of the two.