Power button hold gears to about 5000 rpm so it gives you more responsive driving. Also, with POWER button ON it will downshft very quickly when you press the pedal.
The Hold button will help you not to spin the wheels in slippery conditions. When you move of the line it will start in second gear and it will upshift thru the gears quickly. With HOLD button ON the acceleration will be very slow and the RPM's wont go over 3000 unless it has reached the final gear. Also, when you press the pedal it will not downshift, so no power will kick in and no slipping and sliding will occur.
O/D button lets you select between 3rd and the 4th gear once you're up the speed. OFF = 3rd, ON = 4th
Hope that helps.