overdrive light keeps flashing

bad fuse or bad switch

Driving in overdrive should be done when the driver does not plan on driving fast and jamming the accelerator. Overdrive is like a last gear that helps the vehicle maintain torque at low RPM.

If the overdrive's off light is on that means it's off. If it is off then the overdrive is on.

This is a built in feature to protect you from burning up the transmission. The computer will automatically disengage overdrive when the transmission fluid temperature sensor gets critically high, for instance, when you are trying to tow in overdrive particularly at high speed.

This happened to me in fact hauling a boat several hundred miles on an interstate. I thought the truck should be more than capable of towing it in overdrive but I was later corrected by my dealership. As a rule, if you have something attached to the hitch, turn off overdrive.