O4

O4 may refer to:

LNER Class O4, a class of ex-Great Central Railway 2-8-0 steam tender locomotives
Taff Vale Railway O4 class, a class of 0-6-2T steam tank locomotives
USS O-4 (SS-65), a 1917 United States O class submarine
O-4, the pay grade for the following officer ranks in the U.S. uniformed services:
- Major in the Army, Marine Corps, Air Force, and Space Force
- Lieutenant Commander in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
Tetraoxygen (O₄), oxozone
a class of stars in the stellar classification
Otoyol 4, a highway from Istanbul to Ankara, Turkey
Martin XO-4, an observation aircraft proposed by the Glenn L. Martin Company
$O(4)$ , the orthogonal group in dimension 4

See also