Every now and then my overdrive light comes on saying it is off. Any reason this could be?

 

I would start by checking trans level (in neutral - not park). Also, if it not pretty clear or smells slightly burnt, I would change fluid and filter.

 

Is od actually off or does the light just come on? When does it happen? Are you towing, after you've been driving a while, etc.

 

might be stupid but you may have OD on all the time, and your bulb just works randomly when the bumps hit the filament together. Just a thought.

 

Does the OD turn off and you have the red light in the shape of a transmission come on?

The truck going OUT of overdrive is usually a bad torque converter. The actual reason the OD turns off is because the tranny gets too hot. Basically you cooked your tranny and that is a defense mechanism. I had the same thing happen to my 1996. The torque converter and governor solenoid were replaced and it worked fine.