The power / normal switch changes the shift points of the auto transmission. It's seen in many trucks as a "towing" button or switch. Helps when you're hauling a load uphill, it'll rev up a little higher before shifting to keep power to the wheels under load. If you're driving on flat and not hauling a trailer, you won't see much more power and probably will see a mileage drop.
The overdrive off button simple disables the top gear of the auto transmission. Basically turning the transmission into a 3 speed auto instead of a 4 speed overdrive auto. Not much use for anything I've come across, except maybe when I want to pretend I'm driving standard and I jab the button right before I pull out to pass someone on the highway and push it again once I pull in ahead of him.