A blinking OD light indicates that the transmission is in overdrive mode. This means that the transmission is shifting gears to a higher gear ratio in order to reduce engine speed and save fuel. The overdrive light will blink when the transmission is first shifted into overdrive, and will remain lit until the vehicle reaches its maximum speed for that gear. If the overdrive light is blinking while driving, it means that the transmission is searching for the correct gear ratio to maintain the current speed.