Officers' club
From Halopedia, the Halo wiki
The O-Club is an Officers and Captains club on board UNSC Vessels.
This was Forbidden Territory to all crew on board on board Vessels.
This was a club for Officers and the Captain to enjoy a selection of Cigars and a full bar.
UNSC Gettysburg
The UNSC Gettysburg has the only Mentioned O-Club. But due to the ship being a Frigate.
The Gettysburg also had a Small battle replica of the Actual battle of Gettysburg in 1863.
Larger UNSC Vessels like Cruisers, Carriers and Destroyers would hold O-Clubs in there hulls.
Known Users
Known Spartan users of the Officers Club was Spartan-117, Fred-104, Linda-058, Grace-093 and Will-043 before Operation: FIRST STRIKE.
Other Known users were Vice Admiral Whitcomb, Sgt. Major Johnson, and Lieutenant Harverson.
Sources
Halo: First Strike, pages 286-287