O.S. Meaning of O.S. in English 1. (in prescriptions) the left eye. [ oculus sinister ] 2. Old Saxon. 3. Old School. 4. Old series. 5. (of the calendar) Old Style. 6. See ordinary seaman . Random House Webster's Unabridged English dictionary. Полный английский словарь Вебстер - Random House . 2012