Well, Axel was the exception. Only about half of the Organization was in Castle Oblivion. The members in CoM were Marluxia, Larxene, Axel, Vexen, Lexaeus, and Zexion.
Marluxia and Larxene were killed by Sora
Lexaeus was killed by Riku
Vexen and Zexion were killed by Axel (with help from Riku...