OS

OS may refer to:

O.S. Old Stonyhurst, an old boy of the ancient Jesuit public school, Stonyhurst College
O.S. Engines, a Japanese manufacturer of model aircraft engines
Ocean Science, an Oceanographic Journal published by the European Geosciences Union.
Oculus sinister, meaning "left eye" in eyeglass prescriptions
"Old Style date" (as "O.S."), a date designation indicating derivation from the Julian Calendar, as opposed to "N.S." indicating derivation from the Gregorian Calendar (see Old Style and New Style dates)
Open SEAT, a tennis tournament in Barcelona now known as the Torneo Godó
Open source, a software development model
Operating system, a software that manages the hardware and software of a computer
Order of Santiago, Spanish order dedicated to St James the Greater
Ordinary Seaman (occupation), an unlicensed member of the deck department of a merchant ship
Ordnance Survey, the national mapping agency for Great Britain
Ohtahara syndrome, a brain disorder
Opening snap, a heart sound present in individuals with AV valve stenosis
Outlaw Star, a manga and anime series
Austrian Airlines (IATA designator)
Ossetic language (ISO 639-1 abbreviation)
Ordnance Sergeant
Overall Survival
On-Sight Climbing, the ascent of a rock climbing route on the first attempt, with no falls, without prior knowledge of its specifics beyond its difficulty rating and number of protection points, if a bolted climbing route. An "On-Sight" climb refers to the manner of ascent, and is considered the best style attainable.

os may refer to:

os, a mouth or external opening (from the Latin), also ostium. In particular:
- external os, the external orifice of the uterus
- internal os, the internal orifice of the uterus
- os or ostium, the "mouth" of a coronary artery

Os may refer to: