o my god are you guys that uneducated in what OD is?
OD - Over Drive
Its the tallest gear in which the motor is actually turning slower than the tires. It gives you the most amount of fuel economy, but you have the least amount of power / torque.
You have a button to turn it off when you are constantly climbing and decending hills, or towing for example. The tranny will be constantly trying to get into the highest gear possible (OD) to achieve the best fuel economy, but then need more torque/horsepower to go up a hill etc. Resulting in a ton of upshifting and downshifting, which is needlessly wearing pit your tranny.
Its very simple.
And for the guy saying that you should never have to push the button to get your car out of park, or into any gear for that matter, i believe everyone including me was talking about the crome button that you have to push, while you push the brake pedal to get any auto transmission in the world to move. Some have it on the top, or the front of the lever. ours on the side. Older GM's etc you had to pull the steering column mounted shifter forward then into each gear.
I was unaware that our cars had the little "emergency override" I thought that was pretty much a honda/acura thing.
You'd stick your key into it, then you could move the shifter with the car off.
My 2 cents