when does the overdrive suppose to be on

Overdrive should just operate as a normal operating gear of the transmission. If you have a "O/D OFF" light that is illuminated and cannot be de-selected, or if it blinks - then you need to do some testing to check out the transmission. You would only hit the button to turn "O/D OFF" voluntarily due to towing a load and pulling a lot of hills, or pulling a long steep grade that causes the transmission to shift in and out of overdrive. Otherwise - you would never need that button.