A light on the dash reads "O/D Off". How do I turn it on or off?

Usually there is a button somewhere on the dash that has an O/D button. Check around the shifter to see what each knob says. Sometimes the button is even located on the shifter itself.

If it's the same as the 98 Maxima, there is a 1/2" square button on your shift selector. I think that it should automatically be on when the car is started each time though (you should only have to turn it off, not on), so you might have a transmission problem.

in the 200sx model, its under the transmission shift button, the switch that is used is solid state so it either stays on or off depending on where you left it when you last drove it, i bump mine every so often when shifting to park, but i would suggest you just leave it on, so the light should be off.