{{wiktionary|OS|Os|os}} '''OS''', '''O.S.''', '''Os''', '''O's''', or '''os''' may refer to:
== Computing == * Operating system, system software that manages computer hardware and software resources * Open source (disambiguation) * OpenStack, a software platform for cloud computing
== Medicine == * 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 * For people named ''van Os'' see Van Os
== Places == === Norway === * 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
=== Poland === * Oś, Kluczbork County, a village in Kluczbork County, Opole Voivodeship, Poland * Osiedle (Os.), a Polish term used for housing subdivisions in 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
== 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 * "Os" (''Fringe''), an episode of the television show ''Fringe'' * 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)
== See also == * 0S (disambiguation) * ÖS (disambiguation) * Oz (disambiguation)
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