If OD was off you'd rev higher, ONLY if you were in OD already. If for example you were passing a car (throttle floored) you could flick the OD button all you want & feel no difference

Most every standard tranny has OD as it's top (usually 5th)gear. Some 6-speed manuals have OD for both 5th and 6th gear. My old '79 Stang had a 4-speed manual, and 4th was OD

 

thanks man, helps alot!