For normal drving, you use "D". And when your O/D on meaning you are driving with 4 gears. And O/D on is when the O/D light is not displayed on your front meter board. So for normal daily driving, your O/D light shouldn't be on.
Now, when u need extra power like overtaking another car, than you press the O/D and that's when the gear will shift down from gear 4 to gear 3 to give u the power. Meaning when u press the O/D, it is on and the O/D light in your front meter board should be lighted up. After you've overtook the car infront, than u will have to press the O/D again to activate the O/D.
Yes, no doubt many new auto drivers get mixed up with the O/D.
In conclusion, just remember when u are driving normally, the O/D light shouldn't be on infront of your meter board. Only when u need extra power than u press the O/D. And also when u are going downhill, u do not want to press the brake all the time, than u press the O/D.
I hope I'm not making you "Mong Cha Cha"........