Room above garage smells
If the smell remains after you have fully removed the old carpet and padding and you let the air circulate through room and the window by using a box fan or two then you may very well want to consider replacing the plywood as well. 

The original problem could be caused by cold moist air rising from the garage into the living space above. It could even have been aggravated to some extent by car exhaust and other oil/paint/chemical usage in the garage by the former owner. 

Before you put down new plywood you may want to investigate whether a vapor barrier under the plywood would be a reasonable thing to do. It may very well require a consultation with an expert in your region and such decision would also be dependent upon if there is already a barrier installed such as poly, foil backed insulation or even foil backed drywall on the garage ceiling.