The O2, visually typeset in branding as The O2 arena, is a large entertainment district on the Greenwich peninsula in South East London, England, including an indoor arena, a music club, a Cineworld cinema, an exhibition space, piazzas, bars and restaurants. It was built largely within the former Millennium Dome, a large dome-shaped building built to house an exhibition celebrating the turn of the third millennium; as such, The Dome remains a name in common usage for the venue.

O2, o2, O₂, or O² may refer to:

• Oxygen, by molecular formula (O₂)
• O2, tablets containing Ofloxacin and Ornidazole, made by Medley Pharmaceuticals Ltd in India
• SGI O2, a Unix workstation computer
• UOC O2, UOC's institutional repository