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Oculus Sinister (OS) is defined as the Latin medical term for the left eye. It is the standardized shorthand used by optometrists; ophthalmologists; and opticians to denote:
In Latin; "Sinister" simply means "left." While the word evolved in English to mean something threatening; its medical use remains strictly anatomical. Using Latin terms like OS and OD prevents the confusion that could occur if doctors used local words for "left" and "right" which can sound similar in different dialects.
When an optician reads your prescription; the OS line tells them exactly how to grind the lens for the left side of your frames. This is vital because most people have slightly different prescriptions in each eye. The OS line ensures that the:
Standard clinical practice is to list the right eye (OD) first and the left eye (OS) second. This top-to-bottom order is followed globally to provide a predictable flow for technicians who are making your glasses or contact lenses; reducing the chance of "swapping" the lenses by accident.
Yes; if you wear contact lenses; the OS section of your prescription will also include "Base Curve" (BC) and "Diameter" (DIA) specifically for your left eye. Because the left and right corneas can have different shapes; the OS measurements ensure the left lens fits comfortably and safely on the eye surface.
No; in this context; "Sinister" has no negative meaning. It is strictly the Latin word for "left." Seeing OS on your prescription simply means the doctor is referring to your left eye; and it does not indicate any disease or problem with your vision.
If the numbers on the OS line are larger than those on the OD line; it simply means your left eye requires more correction than your right eye. This is very common and is the reason why eye exams are performed on each eye individually rather than both at once.
If your prescriptions are different; you should never swap your lenses. Doing so will cause blurred vision; headaches; and eye strain. Always check the packaging or the lens case markings to ensure you are placing the OS lens in your left eye and the OD lens in your right.