OP, if you Grand Prix does not have an overdrive button or a d with an o around it then you dont have O/D.

 

my '97 Grand Prix GT never had an overdrive button...but did have overdrive. You could select P R N OD D 2 1

...if I remember right, it was only a year ago

 

You don't need a button to have overdrive. On manual Rangers 5th gear is overdrive. Usually the highest gear is considered the overdrive gear since it brings the revs down for better mileage while cruising the highway. The button will sometimes do other things as Bob stated but in the ends its just an extra gear.