O12

O12 or O-12 may refer to:
 * Curtiss O-12 Falcon, an observation aircraft of the United States Army Air Corps
 * Grand Lake St. Marys Seaplane Base, in Auglaize County, Ohio, United States
 * HNLMS O 12, a submarine of the Royal Netherlands Navy
 * Oxygen-12, an isotope of oxygen
 * USS O-12 (SS-73), a submarine of the United States Navy