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
• , the orthogonal group in dimension 4

See also

• 04 (disambiguation)
• 4O (disambiguation)