From Wookieepedia, the Star Wars wiki.
Oriflamme was the personal Imperial-class Star Destroyer of Grand Admiral Grant. He presumably handed it over to the New Republic when he defected in 6 ABY.
Behind the scenes
- "Grand Admirals: What Have They Got in their Pocketsses?" - "Continuity, Criticisms, and Captain Panaka", Daniel Wallace's StarWars.com Blog