I have a 03 explorer sport track with the O/D light flashing which just started this am. It has 121,000 miles on it and was driving fine with no slipping in the transmission. But I've been told that this is an issue. What do I do to get the light to go out, etc.

 

When the OD lite flashes it means there is a problem in or affecting the transmission.
It could be something simple as a poor connection or loss of a signal or even a defective component. The best way to determine this is to have the code(s) read from the computer , then we go from there. I believe Autozone will do this free of charge as a service. If you choose to do this , get the code numbers . They will most likely tell you what they mean and you can also get a possible explanation from someone here on this site.