Os or OS could signify

• The os is each end of the cervix, internal (to the uterus) and external (to the vagina)
• A municipality of the county Hedmark in Norway, see Os, Hedmark.
• A municipality of the county Hordaland in Norway, see Os, Hordaland.
• The symbol for the chemical element Osmium (Os).
• Operating system (OS)
• Open Source
• Ordnance Survey
• Old Style Julian Calendar
• oculus sinister (OS) - left eye
• Ossetian language (ISO 639 alpha-2, os)
• The Anglo-Saxon ᚩ rune. One of the Aesir
• the IATA code for Austrian Airlines
• A Japanese manufacturer of Model Airplane engines Ogawa Seiki