simply put ..... turning on OD is enabling the final gear for
your car's auto transmission
turn it off and your auto transmission will not use the final
gear
you will understand the concept if you understand a manual
transmission car. why higher fuel consumption? try using only up to
4th gear for a manual transmission car (even on the highway) and
you will understand ... 
or, if you have the luxury of a 6th gear, use only the 1st 5 gears
....