OD and OS are for right eye and left eye respectively. A minus Rx is for correction of myopia or nearsightedness, a positive Rx is for hyperopia or farsightedness. Sphere is to account for any additional astigmatism you may have. Prescriptions cn also 'add' to each lens. This is for the bi-focal part of the lenses. I am nearsighted with an Rx of -1.50 OD -1.75 OS Sphere 160 add +1.00 OD, OS. Even though this is a moderately weak Rx I am still at a big handicap w/o my glasses.