If your car has an OD or overdrive light that is illuminated on the dash, it means there is a problem with the overdrive system. The first thing you should do is check the transmission fluid level and condition. If the fluid looks low or dirty, it needs to be changed. If the fluid looks good, then the next step is to check for any loose or damaged wiring.
If everything looks good so far, the next step is to test the overdrive solenoid. This can be done with a simple multimeter. With the car in park and the engine off, disconnect the overdrive solenoid electrical connector and set your multimeter to ohms mode. Place one probe on each of the two terminals on the connector. You should see a reading of between 10 and 20 ohms if the solenoid is working properly. If not, then it will need to be replaced.