OD and OS are the terms that specialized in eye prescription form, which is used for checking whether the person needs prescription glasses. OD is described as right eye, which is an abbreviation for the Latin term "oculus dexter,"OS is short for "oculus sinister," or left eye. When eye doctor examines your eyes, he/she would see your eyes on the opposite side. In other words, they see your right eye on his/her left and your left eye on his/her right. To ensure the result of eye exam is written in the correct spot on the prescription form, the column of OD is on the left side and OS is on the right of the form.
When you have the eye exam, the doctors may give you the exact prescription of both eyes. About the characters like "OD" and "OS", they are respectively meaning the right eyes and left eyes. From the number of OD, you can get the prescription of right eye. About the number of OS, you can get the left's situation.