O21 may refer to: * ''O21'' (film), a 2014 Pakistani action thriller film *

Hoopa Airport Hoopa Airport is a public airport located on Airport Road next to the Trinity River, one mile (1.6 km) southeast of Hoopa, serving Humboldt County, California, United States. It is mostly used for general aviation. Facilities Hoopa Ai ...

in Humboldt County, California, United States * , of the Royal Netherlands Navy ** , a submarine of this class *

O21 scale O, or o, is the fifteenth letter and the fourth vowel letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''o'' (pronounced ), p ...

, a model railway scale *

Otoyol 21 Otoyol 21 (), abbreviated as O-21, a.k.a. Ankara-Tarsus Otoyolu (), is a completed toll motorway in the Central Anatolia and Mediterranean regions in Turkey. Currently, it connects Otoyol 20 from Gölbaşı, Ankara with the Adana-Mersin Motorwa ...

, a motorway in Turkey *

Oxygen-21 There are three known stable isotopes of oxygen (8O): , , and . Radioactive isotopes ranging from to have also been characterized, all short-lived. The longest-lived radioisotope is with a half-life of , while the shortest-lived isotope is ...

, an isotope of oxygen *

Thomas-Morse O-21 The Thomas-Morse O-19 was an American observation biplane built by the Thomas-Morse Aircraft Company for the United States Army Air Corps. Development The O-19 was based on the earlier Thomas-Morse O-6 biplane. It was a conventional two-seat ...

, an observation aircraft of the United States Army Air Corps

See also