OS

OS, O.S., Os, O's, or os may refer to:

Computing

Operating system, collection of software that manages computer hardware and software resources
Open source (disambiguation)
OpenStack, a software platform for cloud computing

os or ostium, a mouth or mouth-like opening
- per os, meaning "ingestion by mouth"
- external os, the external orifice of the uterus
- internal os, the internal orifice of the uterus
- os or ostium, the opening of a coronary artery
O.S. or OS, oculus sinister, meaning "left eye" in general ophthalmologic or optometric usage, particularly in eyeglass prescriptions (deprecated abbreviation)
Ohtahara syndrome, a brain disorder
Overall survival rate, a cancer survival statistic
Oneiroid syndrome, a dreamlike fantastic delusional state

Os Municipality (Innlandet), a municipality in Innlandet county, Norway
Os Municipality (Hordaland), a former municipality in the old Hordaland county, Norway
Os i Østerdalen, a village within Os Municipality in Innlandet county, Norway
Os, Østfold, a parish in Rakkestad Municipality in Østfold county, Norway
Os Church, a name for several churches in Norway

Esker, also os, a long, winding ridge of sand and gravel in (formerly) glaciated regions
Ordnance Survey, national mapping agency of Great Britain
Austrian Airlines (IATA code OS based on its original name: Österreichische Luftverkehrs AG)
O.S. Engines, a Japanese manufacturer of model aircraft engines

Ohio State Buckeyes, a group of collegiate teams representing The Ohio State University
Os TF, a sports club in Os, Norway
On-sight climbing, an ascent of a rock climbing route on the first attempt
Orioles, nicknamed "O's":
- Baltimore Orioles, a Maryland baseball team
- Charlotte Orioles, a North Carolina baseball team, renamed the Charlotte Knights in 1988
Leyton Orient F.C., an English football (soccer) team nicknamed "O's"
London Olympians, named 1995–2005 as London O's, an American football team based in Greenwich, London, England

Ordinary seaman, an unlicensed member of the deck department of a merchant ship
Ordnance sergeant, an enlisted rank in the U.S. and Confederate armies during the American Civil War era
Old Shirburnian, used by alumni of Sherborne School
Old Stonyhurst, used by alumni of Stonyhurst College
O.S., Order of Santiago, a Spanish order dedicated to St James the Greater
O.S., Order of Sikatuna, the national order of diplomatic merit of the Philippines

"O's", a name for breakfast cereal with O-shaped pieces, cereal similar to , e.g. Oreo O's, Krusty-O's, some kinds of Cheerios — see list of breakfast cereals to find more
SpaghettiOs, a brand of canned ring-shaped pasta in tomato sauce
Newman-O's, cookies made by Newman's Own

Ocean Science (journal), an oceanographic journal
Osmium, symbol Os, a chemical element
"Os" (Fringe), an episode (S3 E16 / #59) of the television show Fringe
Old Style (O.S.) date, indicating use of an earlier calendar (in Anglophone countries, the Julian Calendar), as opposed to "N.S." (new style), usually indicating use of the Gregorian Calendar
Ōs, an Old English word denoting a god in Anglo-Saxon paganism, related to æsir