OS
- 0S (disambiguation)
- Alexander Os
- Austria
- Austrian Airlines
- Baltimore Orioles
- Coronary circulation
- Esker
- Eyeglass prescription
- French language
- Leyton Orient
- List of Old Shirburnians
- List of Old Stonyhursts
- O.S. Engines
- Object sexuality
- Ocean Science (journal)
- Ohio State Buckeyes
- Ohtahara syndrome
- Old Style and New Style dates
- On-sight climbing
- Oneiroid syndrome
- Opavian Silesia
- Open source (disambiguation)
- OpenStack
- Operating system
- Oral sex
- Order of Santiago
- Order of Sikatuna
- Ordinary seaman
- Ordnance Survey
- Ordnance sergeant
- Os (Fringe)
- Os Church (disambiguation)
- Os Guinness
- Os Municipality (Hordaland)
- Os Municipality (Innlandet)
- Os TF
- Os du Randt
- Os i Østerdalen
- Os, Värnamo
- Os, Østfold
- Osiedle
- Osmium
- Ossetic language
- Outback Steakhouse
- Outlaw Star
- Overall survival rate
- Oz (disambiguation)
- Oś, Kluczbork County
- Prompt corner
- Van Os
- ÖS (disambiguation)
- Ōs
- Ōs (rune)