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