A flashing o/d light is a code telling you what is wrong. I don't know what the codes mean, but I think there is a thread or two that address this issue. Not sure what it means when the light stays on and does not flash.
1996 F150 Scab longbed, 5.8L, 4x4, 31x10.5 tires, 212k miles and counting. step bars, low-profile tool box, Euro-clear turn signals & headlights, Euro taillights, vent visors, class III Reese hitch, Tekonsha brake controller.
"when you have a hammer in your hand, everything looks like a nail."