From Halopedia, the Halo wiki
An officers' club, colloquially known as an "O-Club", is a bar and lounge aboard UNSC vessels and bases. This is forbidden territory to all crew on board aboard vessels, save commissioned officers. Officers' clubs located on starships may include a table for dining/conversation, a full bar, and many pictures involving the ship, which may include pictures of the ships captain, the ship's creation, its crew, it in battles, or anything involving its name. For example, the Officers Club on the UNSC Gettysburg contained a daguerreotypes displaying the battle of Gettysburg in the American Civil War, for which the ship is named.
The Gettysburg's O-Club had a massive table of oak, scored with numerous gouges and scorches from a hundred cigars casually set upon its surface. There was a bar stocked with bottles containing a rainbow collection of liquors. It also housed the UNSC gold-fringed blue flag along with photos of past officers and captains of the Gettysburg, as well as tin Civil War daguerreotypes that displayed the battlefield full of charging men, calvalry, and cannons.
UNSC Recreational Facility 6055-NA-A on Neos Atlantis has an officers' club intended for the use of the fleet personnel stationed in the planet's orbital docks. The club features a central bar with screens overhead showing gravball matches, and the exterior window provides an impressive view of the cryovolcano known as the Theran Crown. On October 14, 2553, the officers' club became the site of an incident when an Office of Naval Intelligence Ferret Team consisting of Veta Lopis, Mark-G313, Olivia-G291, and Ash-G099 were targeted by members of Dark Moon Enterprises amid a training exercise.
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