O6

O6 or O-6 may refer to:
 * LNER Class O6, a class of British steam locomotives
 * O6 star, a subclass of O-class stars
 * OceanAir IATA airline designator
 * O-6, a pay grade in the US uniformed services:
 * Colonel in the Army, Marine Corps, Air Force, and Space Force
 * Captain in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
 * the "Occupied 6", an Irish republican euphemism for Northern Ireland
 * USS O-6 (SS-67), an O-class submarine of the United States Navy