The os is the external orifice of the uterus; it is the opening at the tip of the cervix which separates the uterus from the vagina.

Os may also refer to:

• Os, Hedmark, a municipality in Norway
• Os, Hordaland, a municipality in Norway
• Os, Østfold, a parish in Norway
• Ōs, in Old English denoting a god in Anglo-Saxon paganism
• Osmium, a chemical element whose symbol is "Os"
• the Ossetic language (ISO 639-1)
• Os Guinness (born 1941), English author and social critic
• Os du Randt (born 1972), South African rugby player

OS may refer to:

• Open Source software
• Operating system, the software that manages the hardware and software of a computer
• Ogawa Seiki (aka O.S. Engine or O.S.), a Japanese manufacturer of model airplane engines
• Ordnance Survey (OS), the national mapping agency for Great Britain
• Open Seat (aka Open Sabadell Atlantico or OS), a tennis tournament in Barcelona, Spain
• Old Style (OS), a term used in English language historical studies to indicate that a date conforms to the Julian calendar
• The IATA designator for Austrian Airlines
• Outlaw Star, a TV show

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