For every car, their OD (if available) is always on as default. Overdrive means changing the gear up faster as to improve fuel economy. The car will automatically reduce unnecessary revving at the same gear (unless it's the final gear). And if you press the OD button, the display 'OD off' will appear which denotes overdrive function is turned off, you will realise that the rev is more aggresive and more power is delivered but on other hand, fuel economy is not optimised.