O4
# 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 $(O_4),$ 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
