There is no actual button for the OD, but you use the autostick mode. After moving the gear selecter to drive, you can push it to the right between the + and the -. While accelerating, push the gear selecter up (twards the +) and the transmission will shift up and when stopping, you can push it down (twards the -) but it will shift down automaticly. For example if you are going up a hill and want to shift down to have more power, just simply move the gear selecter to the autostick position (the the right) and push it down (twards the -) and it will shift down, and when you want to shift up again just move it up to the + or back to D. So an OD button is not necissary
EDIT- there is a light on the dash next to the D that tells you what number gear you are in