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:

  • 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"
  • Os du Randt (born 1972), South African rugby player
  • Os Guinness (born 1941), English author and social critic
  • Oś, Kluczbork County, Poland
  • Osiedle, a name for a housing estate in Poland
  • "Os" (Fringe), an episode of the television show Fringe

O's may refer to:

  • Baltimore Orioles
  • Leyton Orient