The Osiris, notably, was part of a secret Colonial fleet hidden deep within Cylon territory - the Ghost Fleet Offensive. The fleet consisted of numerous Colonial ships which were reportedly destroyed in various engagements during the first ten years of the First Cylon War.
The ship was assigned to deploy one Dr. Kelly to the ice planet Djerba, a former colonial winter resort destination deep within Cylon territory, a classified mission believed to be key in bringing about the end of the war. Encountering a basestar, its commanding officer decided to engage and deploy its Raptor payload as planned. Due to damage sustained in the opening salvo from the basestar, the Osiris was unable to fire its nuclear weapons due to damage sustained to its outer launch doors. Outnumbered 10 to 1, the commander of the Osiris ordered the ship to be driven into the Basestar; upon impact the commander manually triggered the nuclear weapons, destroying the Osiris and the Basestar.