* OD OFF button: Pressing the "OD OFF" button (usually on the shifter or near it) *disengages* overdrive. This forces the transmission to use a lower gear ratio, useful for towing, steep hills, or when you need more engine braking.
* Automatic engagement: When the "OD OFF" button isn't pressed, overdrive engages automatically once the vehicle reaches a certain speed and engine load.
In short, there's no "on" switch; the default is for it to be on *unless* you press the "OD OFF" button.