Learn · Your prescription, number by number

OD and OS — which eye is which?

OD is your right eye. OS is your left. If you remember nothing else: the one that looks like it should mean "left" does not.

Where the letters come from

They are abbreviated Latin, still standard in eye care worldwide:

OD — oculus dexter

Right eye. Dexter is the same root as "dexterity" — historically the favoured, right-hand side.

OS — oculus sinister

Left eye. Sinister simply meant "left" before it acquired its darker English sense.

OU — oculus uterque

Both eyes. You will see it where one value applies to the pair — often the ADD, or a note about lens type.

The mnemonic that actually sticks: on a prescription the eyes are almost always listed OD first, OS second — and OD/right comes before OS/left alphabetically too. If a form lists them the other way round, read the labels rather than the order.

What you will see instead, abroad

The Latin is not universal, and this trips people up when they move country or compare an old prescription with a new one:

Spanish — OD and OI (ojo izquierdo). Note that OI, not OS, is the left eye — and OD happens to mean the same thing in both languages.
German — R and L, or rechts and links.
English-language practice — increasingly plain R and L, on the reasonable grounds that fewer mistakes get made.

Whose right?

Always yours. A prescription describes your eyes from your own point of view, not the clinician's facing you. The same convention holds for the axis: 180° runs horizontally across your own field of view.

Why getting it right matters more than it sounds

Swapping the two rows is one of the most common transcription errors when a prescription is copied by hand — and if the eyes differ meaningfully, wearing them the wrong way round is immediately uncomfortable. It is also why storing the prescription once, correctly, beats re-typing it each time you order lenses.

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