What is the meaning of "Overdrive (O/D) OFF"? - Automatic Transmission Cars

I have been asked this question over and over again by owners of automatic transmission cars. Myself , i love the manual transmission cars (also know as "stick shift") and this term "overdrive OFF" may not be applicable to the manual cars as you will later find out in this brief article.

Am not going to be too technical and am going to be brief and precise, so i hope this speaks to the layman and immediately clears your doubts about the "O/D OFF" indicator on your car dashboard.

so what is O/D?
O/D simply stands for Overdrive

There are two important components of the O/D that you need to know:

1. The O/D main switch - This is a small button located on the gear selector of your car.

O/D main switch

2. The "O/D OFF" indicator light - The umber light that shows up on the dashboard when the O/D main switch is in OFF position.

O/D OFF indicator light

So here comes the big questions; what does O/D do? When do i need it? what is the default position of the O/D switch? 
Like i promised, the answer is short and precise.

Overdrive allows the car to operate in the highest gear in the transmission (Usually gear #5 or gear #4 for cars that only have 4 gears). This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When O/D is OFF, the automatic transmission shifting is limited to the lower gears (gears #1, #2 & #3).

By default, "O/D is ON"; That's the O/D switch button is pressed and the O/D indicator light on the dashboard is NOT showing (means O/D ON).
"O/D is OFF" when the O/D switch button is released and the O/D indicator light on the dashboard is lighting.

O/D switch positions

You need "O/D ON" if; you are on a highway and driving at higher speeds, usually above 70 km/h. (allows the car to operate at higher gears and this is friendly to the engine and is fuel efficient).

You need "O/D OFF" if; you are on a bad road, going up a hill, carrying a heavy load, driving in slow city traffic, going down the hill and you need engine braking, or if you need quick acceleration for example during an overtake.

If you have any questions about the O/D function, feel free to comment below, otherwise happy driving :-)