O21 may refer to:
O21 (film), a 2014 Pakistani action thriller film , of the Royal Netherlands Navy , 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