When do you turn the overdrive off?

When you're carrying a heavy load or going uphill and the transmission is not shifting down to the correct gear (vehicle accelerates slowly or loses speed)

Also to allow the engine to slow the car when going down a steep hill - less wear/strain on your brakes. The effect it will have in slowing the car depends on how fast you are traveling, you may need to shift down a gear (From D into 3, for instance) instead of disengaging your O/D if you are traveling slowly and want to let the car 'hold you back' instead of using your brake excessively.