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), USS ''O-4'' (SS-65), a 1917 United States O class submarine * O-4, the Uniformed services pay grades of the United States, pay grade for the following officer ranks in the U.S. uniformed services: ** Major (United States), Major in the Army, Marine Corps, Air Force, and Space Force ** Lieutenant commander (United States), 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 * , the orthogonal group in dimension 4

See also