In Days, Roxas uses the Gear system to wield other Keyblades, designed after fellow Org. members weapons.
Oathkeeper and Oblivion are symbolic of his connection with Sora. Oathkeeper represented Sora's promise to Kairi, Oblivion representing Sora and Riku's friendship. Those two being the strongest of Sora's memories, part of them carried over to Roxas, who can wield the Keyblades formed from them.