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