[wiswind]

If the O/D light on, that is an indication that the Overdrive is disabled. There is a button on the end of the shift lever to enable or disable the overdrive.
However, the light should either be ON or OFF all the time....not just over a certain speed.

Mine glows very dimly when the headlights are on.

If the O/D light is FLASHING....it is indicating that there is a problem with the transmission. You will need to get those codes read. A dealer, or some shops can read those codes. It is like a "Check Engine Light"....only it is for just the transmission.