From Halopedia, the Halo wiki
HIGHCOM Armory Omega was one of many weapon caches placed on Reach in times of emergency.
During the Fall of Reach, HIGHCOM Headquarters had sent reinforcement supplies from the omega armory. It contained special weapons created by the Office of Naval Intelligence, such as the NOVA Bomb, as well as many standard infantry weapons and vehicles. This cache was recovered by Red Team during the beginning of their defense of Reach's Orbital Defense Generators. It is unknown what happened to the cache's supply of weapons, minus the NOVA, after the Covenant began glassing Reach.<ref name="HFSC3">