O.S. (left eye (L. oculus sinister))

O/E (on examination)

O/R (oral, rectal)

O&E (observation and examination)

O&G (obstetrics and gynecology)

O&P (ova and parasites)

O2 (oxygen)

O2Sat (Oxygen saturation (percent))

O3 (Omega-3)

O3 (ozone)