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 type of computer programming
- Original Oratory, a competitive event in high school forensic competitions
- SkyWest, IATA code OO
- Oakley, Inc., NYSE stock ticker symbol OO
- Oscillation Overthruster (OO), a Super Flag from the video game BZFlag
- OO, the depot code for the carriage shed at Old Oak Common TMD
See also
- 00 (disambiguation)
- OOo, OpenOffice.org
