O3 may refer to:
Science and technology Ozone (O3), an allotrope of oxygen , the ozonide anion O(3), the 3-dimensional orthogonal group in group theory O3 star, a subclass of the O-type star stellar classification Haplogroup O3 (Y-DNA), a Y-DNA Haplogroup OpenAI o3, an artificial intelligence model
Music O3 (album), by Sunscreem, 1993 O3, a 2008 album by Son of Dave O3 Trilogy, a series of albums by Dominici
Military O-3, a uniformed services pay grade of the United States for the following: Captain (United States O-3), in the Army, Marine Corps, Air Force, and Space Force Lieutenant (navy), in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps USS O-3 (SS-64), a 1917 United States O class submarine
Transportation Oldershaw O-3, a glider Otoyol 3, a Turkish motorway from Edirne to Istanbul LNER Class O3, a class of British steam locomotives
Other uses Oskarshamn 3, unit 3 at Oskarshamn Nuclear Power Plant in Sweden
See also 03 (disambiguation) Ö3, Austrian radio station 3O (disambiguation)