O3

O3 may refer to:


 * O3, the molecular formula for ozone
 * O(3-), the ozonide anion
 * O3, the 1993 debut album by the techno band Sunscreem
 * O3 A Trilogy, a 2008 concept album trilogy by progressive metal band Dominici
 * O3 star, in stellar classification, a subclass of type O stars
 * O-3, the pay grade for the following officer ranks in the U.S. uniformed services:
 * Captain in the Army, Marine Corps, Air Force, and Space Force
 * Lieutenant in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
 * Haplogroup O3 (Y-DNA), a Y-DNA Haplogroup
 * Oldershaw O-3, glider
 * USS O-3 (SS-64), a 1917 United States O class submarine
 * Otoyol 3, a Turkish motorway from Edirne to Istanbul
 * Oskarshamn 3, unit 3 at Oskarshamn Nuclear Power Plant in Sweden
 * O3 (album), a 2008 album by Son of Dave
 * LNER Class O3, a class of British steam locomotives