OD on a prescription stands for “oculus dexter,” a Latin term meaning “right eye.” You’ll see it on eyeglass and contact lens prescriptions to label the measurements for your right eye. It’s one of several Latin abbreviations eye care providers use, and once you know what each one means, reading your prescription becomes straightforward.
OD, OS, and OU Explained
Your eye prescription will almost always have two rows of numbers, one for each eye. OD (oculus dexter) marks the right eye, and OS (oculus sinister) marks the left eye. Some providers skip the Latin entirely and just write “RE” for right eye and “LE” for left eye. A third abbreviation, OU (oculus uterque), means both eyes, but it rarely appears on prescriptions since most people need slightly different corrections for each eye.
One thing that trips people up: OD refers to YOUR right eye, not the right side as someone looks at you. When you’re reading your prescription, the OD line is always about the eye on your right.
What the Numbers Next to OD Mean
The OD label by itself just tells you which eye the numbers apply to. The actual correction is described by several values listed across that row.
- SPH (Sphere): This is the main strength of your lens, measured in diopters. A minus sign means you’re nearsighted, and a plus sign means you’re farsighted. The further the number is from zero, the stronger your prescription.
- CYL (Cylinder): This corrects astigmatism, which is when your cornea is slightly oval-shaped rather than round. If this box is empty, you don’t have astigmatism in that eye.
- Axis: This is a number between 1 and 180 that describes the angle of your astigmatism. It always appears alongside a CYL value and tells the lab how to orient the lens.
- Add (Addition): If you need bifocals or reading glasses, this number represents the extra magnifying power for close-up vision. It typically falls between +0.50 and +3.50 and is the same for both eyes, so it usually appears only once on the prescription.
- Prism: This value corrects eye alignment issues by shifting the image your eye sees. It’s measured in prism diopters and comes with a direction (up, down, in, or out). Most prescriptions don’t include prism unless you have a specific alignment condition.
OD on a Medication Prescription
Outside of eye care, OD occasionally shows up on medication prescriptions, where it can mean “once daily.” This double meaning has caused real problems. The Institute for Safe Medication Practices lists “o.d.” as an error-prone abbreviation because pharmacists and nurses have confused “once daily” with “right eye,” leading to oral medications being incorrectly administered as eye drops.
Because of this risk, most hospitals and pharmacies now avoid using “o.d.” for dosing instructions altogether. If you see OD on a prescription from your regular doctor or a pharmacy label, it almost certainly means “once daily,” but the safer, more modern practice is to spell out “daily” instead. If you’re ever unsure whether OD on your prescription refers to your right eye or a dosing frequency, the type of provider who wrote it is your best clue: an optometrist or ophthalmologist means right eye, while a general practitioner likely means once daily.
Glasses Prescriptions vs. Contact Lens Prescriptions
Even though both types of prescriptions use OD to label the right eye, they aren’t interchangeable. A contact lens prescription includes additional measurements that a glasses prescription doesn’t have, specifically the base curve (the curvature of the lens to match your cornea) and the diameter of the lens. It also specifies the manufacturer, brand, and an expiration date. The power values themselves can differ too, because contact lenses sit directly on your eye while glasses sit about 12 millimeters in front of it.
If you have a glasses prescription and want to order contacts, you’ll need a separate contact lens fitting. The OD and OS labels work the same way on both, but the numbers won’t necessarily match.