O21 may refer to:
- O21 (film), a 2014 Pakistani action thriller film
- Hoopa Airport in Humboldt County, California, United States
- O 21-class submarine, of the Royal Netherlands Navy
- HNLMS O 21, a submarine of this class
- O21 scale, a model railway scale
- Otoyol 21, a motorway in Turkey
- Oxygen-21, an isotope of oxygen
- Thomas-Morse O-21, an observation aircraft of the United States Army Air Corps
See also
- 021 (disambiguation), with a zero in place of the letter O
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