O21
O21, O 21 or O-21 may refer to:
- O21 (film), a 2014 Pakistani action thriller film
- O21, FAA identifier for Hoopa Airport
- O 21-class submarine, a class of submarines built for the Royal Netherlands Navy
- HNLMS O 21, a submarine of this class
- O21 scale, a set of model railway standards for narrow gauge railways
- Otoyol 21, a motorway in Turkey
- Thomas-Morse O-21, an American observation biplane