Most of the time you want to leave the O/D alone. When you start your car it will automatically be in O/D.
When you turn it off, it keeps the transmission from shifting higher then 3rd gear. This gives you better control when you need it.
You can use it:
1. If your towing anything
2. You are climbing a steep hill.
3. If you are going down a steep hill, or if the roads are snowy and slippery and your going down any size hill, then turn it off.
According to Samarins
"Generally speaking, overdrive (O/D) is the highest gear in the transmission. On most cars the automatic transmission has 3 speeds and Overdrive (forth speed). Overdrive allows the engine to have less rpm with higher speed in order to have better fuel efficiency. When you switch it on, you allow the transmission to shift into overdrive mode after the certain speed is reached (usually 30 - 40 mph depending on the load). When it
〉 Answered on Dec 5th, 2007 by Amy Mattinat, Owner and Author at Auto Craftsmen Ltd