O/D stands for "overdrive", which is the 4th gear of the transmission. If the light is on and it is not desired (there is a button that turns off overdrive that would illuminate the light as well), then you have a fault recognized by the transmission and processor. This would require a diagnostic test to be performed.
Of course - I'm thinking with a 5/60 base warranty and 10/100 powertrain warranty to the original owner - you gotta have coverage - yes? See your local dealership with your concern. Good luck!