# O2

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

## Science and technology

* Dioxygen (O$_2$), the common allotrope of oxygen
* Oxide (O$^{2−}$), an ion
* Superoxide (O−2), an ion
* Dioxygenyl (O+2), an ion
* Doubly ionized oxygen (O$^{2+}$), 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

## Places

* 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

## Arts, entertainment and media
### Film

* 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

### Music
#### Albums

* O$_2$ (FireHouse album) (2000) album of the rock band FireHouse
* O2 (O-Town album) (2002) album by American boy band O-Town
* O$_2$ (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

#### Songs

* "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

### Other arts, entertainment and media

* 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

## Businesses and organisations

* 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

## Vehicles and military

* 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

### Pay grade

* 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