O3

O3 may refer to:

• Ozone (O₃), an allotrope of oxygen
• O³⁻, 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

• O₃ (Sunscreem album), 1993
• O3 A Trilogy, a 2008 concept album trilogy by progressive metal band Dominici
• O3 (Son of Dave album), 2008

• 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

• Oldershaw O-3, a glider
• Otoyol 3, a Turkish motorway from Edirne to Istanbul
• LNER Class O3, a class of British steam locomotives

• Oskarshamn 3, unit 3 at Oskarshamn Nuclear Power Plant in Sweden