OA, O.A., Oa, or oa may refer to:

• Old Alleynian, an old boy of Dulwich College
• Oxford Academy (California), #2 ranked public high school in the United States

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 Jordan's "energy duplicate" so they could hear of his origin.

Ovada (Uà and Guà in Ligurian, Ovà in Piedmontese) is a comune (municipality) of 11,613 inhabitants in the Province of Alessandria in the Italian region Piedmont, located in the "Alto Monferrato" area about southeast of Turin and about south of Alessandria.