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I had the same problem with my old E-350. Try this:
Remove one of the battery cables for 5 minutes ore more. Then connect it back, start the engine, drive the car nice and easy up to O/D, drive in O/D for 2 minutes ore more before you stop the car. Turn of the ignition. When you start next time, will hopefully the O/D light be turned-off.
This procedure helps me for a couple of days, and then the problem went back. If this happen again take the car to a dealer for error code reading.
All that does is get rid of the blinking O/D light, it doesn't remove the code or problem. I had to have my torque converter replaced on mine. I highly recommend you get that code checked.