I have a 2000 Ford Explorer, 4WD, with about 65000 and I am having some troubles shifting from first to second, and second to third. The problem only occurs after a few minutes of driving. After the car has warmed up, it shifts hard from first to second and even harder from second to third. After about another 15 miles of driving, the 'Over Drive Off ' light starts blinking, and it starts shifting better, but slower. After my car is off for about an hour or so,it will shift normal again when I drive off (again, only for a few minutes.) Also, does anyone know where I can find the oxygen sensor on a 2000 explorer? Please help!

It may have more than one O2 sensor, just follow along the exhaust system and you will find it. As for the Trans problem.... How often do you change the oil in it? Does it have enough oil in it? It may need a rebuild. C >

When was the last time you serviced the transmission? Have you checked the transmission oil and if so how dirty is it ?
I am no expert but I bet the issue you discribed has something to do with th etransmission.