O21

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