Oa is a fictional planet that lies at the center of the DC Comics universe. Since its inception, Oa has been the planetary citadel of the Guardians of the Universe and the headquarters of the Green Lantern Corps. It first appeared in Green Lantern vol 2 #1, when the Guardians summoned Hal's 'energy duplicate' so they could hear of his origin.

Read more about Oa:  History, Green Lanterns Corps Headquarters, Book of Oa