Just copy and paste here
The overdrive (O/D) is like the 4th gear for a auto car. The O/D is usually 'ON' meaning the auto trans will change from 1st to 4th gear under normal driving.
When the O/D is OFF (usually shown with the words "O/D off" in orange in the instrument cluster. The 4th gear will be 'disabled'.
"O/D OFF" is usually used for engine brake (eg. going down a hill over a long period of time so as not to 'cook' your brakes. You may also shift to 'L' or '2' (meaning 1st and 2nd gear respectively) for engine brake if the downslope is even steeper.
Another use of the "O/D OFF" is for overtaking (like shifting to 3rd gear in a manual trans car). After you have done your overtaking, you can then switch the O/D switch to 'ON' which alllows the auto trans to change to 4th gear.
For driving in SG, the O/D should be always be ON which is also the 'default' setting for the auto trans. As for overtaking, usually if you accelerate hard enough, the auto trans will downshift by itself and change back to 4th gear when you ease off the accelerator.