• 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 programming (OOP), 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
• 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
• Ozzy Osbourne, musician
• Oo is the Tagalog word for "yes."

See also

• 00 (disambiguation)
• OOo, OpenOffice.org