The Officer's Club is a lounge on Galactica where pilots and other officers can meet to relax and have a drink.
Each table has a bell in its proximity, so that officer's can get the attention of their fellow patrons. Sometimes this means notifying the patrons that a drink is "on them" (TOS: "Lost Planet of the Gods, Part II").
Cain and Cassiopeia meet in the Officer's Club (which forces Sheba to leave in disgust). Sheba and Bojay also meet Cain here and express their support of him, even to the point of mutiny (The Living Legend).