OO or oo may refer to:
- Oo (digraph), a digraph in orthography
- Oô, a commune in France
- OO gauge, a 1:76.2 modelling scale in model railroading
- ‘Ō‘ō, an extinct bird of the genus Moho
- object-oriented, a computer programming paradigm
- Official Observer, an appointed amateur radio operator
- Original Oratory, a competitive event in high school forensic competitions
- SkyWest Airlines, IATA code OO
- Belgium, OO (Oscar, Oscar), ICAO aircraft registration prefix
- Oakley, Inc., NYSE stock ticker symbol OO
- OO, the Newtwork Rail depot code for the carriage shed at Old Oak Common TMD
- O.o or o.O, emoticon used in online chat (appears as two eyes) to describe confused or scared
- Oo, a Scottish term for wool that was first noted by Allan Ramsay in the glossary to his poems in 1721