O2, O-2, o2, or similar orthography may refer to:

• Dioxygen (O₂), the common allotrope of oxygen
• Oxide (O²⁻), an ion
• Superoxide (O−2), an ion
• Dioxygenyl (O+2), an ion
• Doubly ionized oxygen (O²⁺), an ion
• O(2), the 2-dimensional orthogonal group in group theory
• O2, an EEG electrode site according to the 10–20 system
• SGI O2, a Unix workstation computer
• O2, a class of O-type star
• Orbital O2, a tidal power turbine in Orkney, Scotland

• The O2, an entertainment district in London, England
  • The O2 Arena, the arena within The O2
• O2 Arena (Prague) or Sazka Arena, an arena in Prague, Czech Republic
• O2 Centre, an indoor shopping and entertainment centre on Finchley Road, London, England
• O2 Residence, a part of the Jumeirah Lake Towers in Dubai, United Arab Emirates
• O2 World (Berlin), an indoor arena in Berlin, Germany
• O2 World (Hamburg), an indoor arena in Hamburg, Germany
• Otoyol 2, a motorway in Turkey called "O2"
• The O2 (Dublin), the former name of 3Arena
• Ring 2 (Aarhus), a road in Aarhus, Denmark, which appears as "O2" on signs
• Ring 2 (Copenhagen), a road in Copenhagen, Denmark, which appears as "O2" on signs

• Oxygen (2021 film) or O2, an American-French survival thriller film in production
• O2 (2020 film), an Estonian-Latvia-Lithuanian-Finnish historical spy thriller film
• O2 (2022 film), an Indian Tamil language survival thriller film
• O2 (2024 film), an Indian Kannada language medical thriller film

• O₂ (FireHouse album) (2000) album of the rock band FireHouse
• O2 (O-Town album) (2002) album by American boy band O-Town
• O₂ (Tonéx album) (2002) album by Gospel singer Tonéx
• O2: Avalon Remixed, a 2002 album by Avalon
• O2 (Son of Dave album), 2006 album by Son of Dave

• "O2", a song by Orange Range from Panic Fancy
• "O2", a 2002 song by Sleater-Kinney from One Beat
• "O2", a song by Suho from Self-Portrait

• Ö2, a radio service for Austria and South Tyrol
• O2, a character in Kirby 64: The Crystal Shards
• UOC O2, institutional repository of the Open University of Catalonia

• O2 (brand), a global brand name owned by the Spanish telecommunications company Telefónica
  • O2 Store, a chain of retail stores owned and operated by Telefónica Europe, with regional subsidiaries:
  • O2 (brand), Telefónica mobile network global brand name
  • O2 (Ireland), merged into Three Ireland
  • O2 (UK), a joint venture with Liberty Global
  • O2 Czech Republic
  • Telefónica Germany
  • O2 Slovakia
• O2 Academy, a chain of music venues
• O2 Wireless USA or H2O Wireless, a prepaid wireless service by Locus Telecommunications
• O2TV, a Russian independent TV channel

• Cessna O-2 Skymaster, a military twin-engine light aircraft
• Douglas O-2, a military single-engine observation biplane
• O 2-class submarine, a class of submarines of the Royal Netherlands Navy
• Oldershaw O-2, a glider
• SP&S Class O-2, a 1910 steam locomotives class
• USS O-2 (SS-63), a 1918 United States O class submarine
• GNR Class O2, a class of British steam locomotives classified O2 by both the GNR and the LNER

• O-2, a uniformed services pay grade of the United States:
  • First lieutenant (United States) in the Army, Marine Corps, Air Force, and Space Force
  • Lieutenant (junior grade) in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps

• 02 (disambiguation)
• Oz (disambiguation)
• 2O (disambiguation)
• 2Q (disambiguation)
• Q2 (disambiguation)