Others
- Old Stone Age (Paleolithic)
- Osa (plant genus), a genus of plants in the Rubiaceae family
- Osa, nickname of former Takarazuka Revue actress Sumire Haruno
- Office of the Science Advisor of the United States Environmental Protection Agency
- Office of Secret Actions, a fictional government department in Return to Castle Wolfenstein
- The Office of Special Affairs, a controversial department of the Church of Scientology
- Official Secrets Act
- Online Savings Account
- Open skies agreement
- Operación San Andrés
- Operation Save America
- Opportunistic Spectrum Access (AKA Smart radio, see also related article Frequency-hopping spread spectrum)
- Optical Spectrum Analyzer
- Optical Supply of Asia
- The old airport code for Osaka International Airport
- Oscilloquartz SA
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