when you turn off O/D your engine only shifts up to third gear (for four speed automatics, not sure for the 5 or 6 speed automatics), this is usefull when towing something heavy or during stop and go situations where a fourth gear is not necessary. But for normal driving turn on the O/D to be able to use all the gears and save on fuel.
kung mabagal yung takbo sa city, let's say matrapik, turning off O/D off won't have any difference because your tranny will be using only 1st and 2nd gear mainly anyway