OS may refer to: O.S. Engines, a Japanese manufacturer of model aircraft engines Oculus sinister, meaning "left eye" in eyeglass prescriptions Old Style, a term used in historical studies to indicate that a date conforms to the Julian calendar Open SEAT, a tennis tournament in Barcelona now known as the Torneo Godó Open source, a software development model Operating system, the software that manages the hardware and software of a computer Ordinary Seaman (occupation), an unlicensed… (More on Os)