i quote this from some other web.
"[FONT=arial, Arial, Helvetica]Strictly speaking, the Overdrive switch does not control an overdrive at all. It is simply an electrical switch that forces the PCM to shift from 4th gear to 3rd, to provide more engine torque for overtaking. [/FONT][FONT=arial, Arial, Helvetica]The OD light is controlled by the PCM, not the OD switch on the gearchange lever, and it is the PCM which turns on the light. It follows that, if the PCM detects a problem with the gearbox, which is fully electronic, then it can blink the OD light.[/FONT][FONT=arial, Arial, Helvetica]"
[/FONT]there are many causes that will trigger O/D light blinks.
to name a few: ATF level, ATF temperature, input/output shaft speed sensor, torque converter, shift solenoid,etc...
but 1st, check ATF level, maybe it's time to flush and replenish with new ATF. if problem still persist, then it might not be the GB itself, but might be due to the TCM (Transmission Control Module) or PCM (Powertrain Control Module) that controls all the input. this means maybe faulty control module.
the best way is to bring to workshop, check do the diagnostics on OBD-II to retreive the error code.