O/D is called overdrive
This button is intended for people who is driving on a fast speed.
O/D is just the 4th and 5th gear of a standard car!
If you turn it off.. it stays in the 4th gear and gas-save mode while driving not too fast ( at most 100 )
If you turn it on. it will shift to 5th gear.. and for you if you like to drive fast.
you can try it the following: turn it on and drive to 130 and turn it off immediately.. you will see the differnce.. becareful, its extremely dangerous and will get a ticket or hurt someone if you are on this speed