OS stands for 'Oculus Sinister' and is the latin phrase for 'Left Eye'.
OD stands for 'Oculus Dexter' and means 'Right Eye'.
When you receive a glasses or contact lens prescription, it will typically include separate values for the right and left eyes, and these abbreviations help ensure that the correct prescription is applied to each eye. Additionally, prescriptions may include other information, such as the spherical power, cylinder power, axis, and sometimes a base curve for contact lenses, or a pupil distance for glasses, depending on the type of correction needed for each eye.