The overdrive gear is a gear for fuel economy, typically seen on four speed automatics. In this case, the overdrive gear would be the fourth or top gear.

If you’re on the highway and you push the overdrive button, you should feel and hear your car downshift. Pushing this button kicks you into third gear, which is better for acceleration. This can be useful on hills, for instance.

Some dashes illuminate “overdrive” when the button is pressed, which is confusing. Other vehicles say “O/D Off”, which is a more accurate label in my opinion. Pushing the button essentially locks the car out of top gear so it’s not shifting back and forth a bunch if you’re trying to maintain a constant speed on a hill.