Why would the overdrive on/off light blink - 2000 Ford Expedition

By the sounds of it that could be the solenoid that operates the overdrive that has malfunctioned or there's a bad earth or a broken fuse - its electrical though that's for sure -- you may need to see a mechanic for that one unless your good at diy :)
Hope you get it going :)

check the transmission fluid, when it is low it can make the overdrive light blink

Have the vehicle scanned.The overdrive light is like the check engine light for the transmission. There are transmission codes in the ecm if the overdrive light is blinking.

My Overdrive light keep blink?

This blinking light indicates a problem. You will have to have it scanned for trouble codes to find out
what it is.

sounds like a bad switch or sensor

if the overdrive light is blinking then there is a problem with the TCM transmission control module. if you have access to a scanner, pull the codes, that will lead you on the path to what is wrong. if you dont you will have to take it for service.

The overdrive light blinking indicates a code has been stored in you computers transmission control module, the cost will depend on the problem, some have had problems with the torque convertor clutch slipping, could be more major, get the code pulled out and I can provide you with a more complete answer. -Jared

OVERDRIVE ENGAGEMENT IS TIED TO THE SPEED SENSOR SIGNAL, THE SYSTEM WILL NOT GO INTO OD UNTIL THE CAR REACHES A CERTAIN SPEED, IN MOST CASES AROUND 35-40 MPH, THE SPEED SENSOR SIGNAL TELLS THE ECU WHEN THE CORRECT SPEED HAS BEEN REACHED.

The blinking overdrive button is telling u that a serious fault has occurred in the electronic controls or a mechanical fault such as excess slippage is occurring.