# OS

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

## Computing

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

## Medicine

* os, a bone in french
* os or ostium, a mouth or mouth-like opening
** external os, the external orifice of the uterus
** internal os, the internal orifice of the uterus
** per os, meaning "ingestion by mouth"
** 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

## People

* Alexander Os (born 1980), a Norwegian biathlete
* Os du Randt (born 1972), a South African rugby player
* Os Guinness (born 1941), an English author and social critic

## Places
### Norway

* Os, Innlandet, a municipality in Innlandet county, Norway
* Os, Hordaland, a former municipality in Hordaland county, Norway
* Os, Østfold, a parish in Rakkestad municipality in Østfold county, Norway
* Os Church, a name for several churches in Norway

### Poland

* Oś, Kluczbork County, a village in Kluczbork County, Opole Voivodeship, Poland
* Osiedle (Os.), a Polish term used for housing subdivisions in Poland

### Germany

* Opavian Silesia or "Upper Silesia", a portion of Silesia in the German Empire that opted to remain in Germany's Weimar Republic and Nazi Germany until World War II. After the War, it was divided between Czechoslovakia and Poland.

### Other places

* Os, Värnamo, a village in Värnamo Municipality, Småland province, Sweden
* OS, ICAO code for airports in Syria
* OS, WMO country code for Austria

## Religion

* Ōs, an Old English word denoting a god in Anglo-Saxon paganism, related to æsir
* 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

## Science

* Ocean Science (journal), an oceanographic journal
* Osmium, symbol Os, a chemical element

## Businesses

* 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

## Entertainment

* "Os" (Fringe), an episode of the television show Fringe
* Outlaw Star, a manga and anime series
* OS, opposite prompt or stage right in a theatre

## Sport

* 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
* Baltimore Orioles, an American baseball team nicknamed "O's"
* Leyton Orient, an English football (soccer) team nicknamed "O's"

## Titles

* 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

## Other uses

* Esker, also os, a long, winding ridge of sand and gravel in (formerly) glaciated regions
* Ōs (rune) (ᚩ), a rune of the Anglo-Saxon fuþorc
* 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
* Ossetic language (ISO 639-1 abbreviation OS)
* Object sexuality, sexual attraction to inanimate objects
* Oral sex
* On Sheet, a record of a train passing a location