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
This disambiguation page lists articles associated with the same title formed as a letter–number combination.
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