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

• 021 (disambiguation), with a zero in place of the letter O

Topics referred to by the same term

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