O.S means Left Eye.
O.D means Right Eye.
Sphere: it indicates the strength of your prescription. Distance prescriptions will usually have a (-) before the number. Reading prescriptions will usually have a (+) before the number.
Cylinder: this indicates the amount of lens power for astigmatism. If nothing appears in this column, either you have no astigmatism, or your astigmatism is so slight that it is not really necessary to correct it with your eyeglass lenses.
Axis:this describes the lens meridian that contains no cylinder power to correct astigmatism. The axis is defined with a number from 1 to 180. The number 90 corresponds to the vertical meridian of the eye, and the number 180 corresponds to the horizontal meridian.
Prism: Prism is prescribed to correct a muscle imbalance between the two eyes so it should help make your vision more comfortable.
Base: the direction of the prim power.
Add: it is short for Reading Addition, is the additional correction required for reading. It can be used to make either reading glasses, bifocal glasses or multifocal glasses.
PD: it is the distance between pupil centers.