In the United States, you need a written prescription to buy contact lenses. The prescription form will list measurements for each eye under columns labeled "OD" and "OS."
OD is an abbreviation for the Latin term “oculus dexter,” or right eye. OS is short for “oculus sinister,” or left eye.
When eye-care providers examine you, they see your right eye on their left and your left eye on their right. To ensure that they write the measurements in the correct spot on the prescription form, the column for your right eye, or OD, is on the left side of the form, and OS is on the right.
Under the OD and OS columns, your prescription will list six numerical measurements for each eye: PWR (corrective power), BC (curve of the back of your eye), DIA (diameter), CYL (cylinder), AXIS and ADD. Both CYL and AXIS are measurements to correct for astigmatism, while ADD identifies a second area of focus for bifocal contacts.
A positive number in the PWR section under OD and OS indicates farsightedness, while a negative one signals nearsightedness.
If the same measurement applies to both of your eyes, your provider may use the abbreviation OU. This is short for “oculus uterque,” or each eye.