OO
OO or oo may refer to:

Science and technology
 ʻŌʻō, an extinct bird of the genus Moho
 Object-oriented programming, a computer programming paradigm
 O.O (also O.o or o.O), an emoticon to represent two eyes and a nose or mouth

Transportation
 SkyWest Airlines (IATA code: OO)
 Belgium (aircraft registration prefix: OO)
 Old Oak Common TMD, a carriage shed (Network Rail depot code: OO)

Places
 Oô, a commune in France
 Oo, Indonesia

Art and entertainment
 OO (EP), album by South Korean singer Zion.T
 OO, the production code for the 1967 Doctor Who serial The Ice Warriors

Other uses
 Oo (digraph), a digraph in orthography
 ‘O‘o stick, a traditional Hawaiian digging bar
 Original Oratory, a competitive event in high school forensic competitions
 Oakley, Inc. (NYSE stock ticker symbol: OO), subsidiary of Italian company Luxottica
 OO gauge, a 1:76.2 modelling scale in model railroading
 Official Observer, an appointed amateur radio operator

See also
 Double-O, producer and half of hip-hop duo o in the Hallo
 Infinity (∞)
 00 (disambiguation)
 O0 (disambiguation)
 OOO (disambiguation)
 ᅇ, an obsolete unit of the Hangul alphabet