When your car uses Overdrive Off, you and the vehicle are safe. The overdrive feature makes it easier to navigate through rugged mountainous terrain. You need to move slowly and use traction to ensure the safety of yourself and others.
However, Overdrive Off will not be suitable for driving on the highway. You need a high speed, so O/D should be turned on to help you limit unnecessary fuel consumption.
When Overdrive is on, the noise from the engine reduces, the driving speed is faster and smoother. In contrast, Overdrive in Off mode speed will be average, consuming more fuel.