what does it mean if a dashboard light is on that says OD off
It means just that, it is Off. By pressing the button on or near the shifter, you disable Overdrive (OD). This keep the vehicle from shifting into the higher gear when you don't need it while driving around town and less than 40MPH. This help the transmission from hunting in and out of OD around the 40MPH putting wear on the transmission.
Including up steep grades, it will also do just exactly what Tom said: hunt for the optimum be it O/D or the next gear down, sometimes shifting back and forth unnecessarily. If you travelling out on the open road, say, with cruise control on at 70, then O/D lowers engine RPM's for better fuel economy and less wear. I do not mean to speak for Tom, he knows more than I do.but neither of us are suggesting you ignore it. Push that button and see if the light goes out and you detect it shifting up at about 40 or so . If you do not know where the button is check your owners manual, yeah, I know a '93. probably long gone...you can Google it