Ford transmission slipping and OD Light blinking

[erinmc]

My transmission is slipping and my over drive light is blinking

[wildman]

Year of the truck would guide a little about the internal workings, but slipping is never good.
Have you checked the codes, because sometimes an error for solenoids will show.
The speed sensor for some models goes rogue and affects shifting, causing the trans to fail.

Slipping is usually caused by two things, fluid under required pressure or the mechanical clutch disks worn.

I would start by changing the fluid and filter in the transmission and see if it fixes the problem. It's not expensive if you do it yourself, and sometimes fixes the problem. While you're under, look at the transmission cooler lines to make sure they're not pinched causing problems.
If it works after, great but if not you're most likely looking at torque converter, clutches, or the transmission as a whole.