for a normal 4 speed auto tranny car

when u put it at D with OD on, it will uses all the 4 gears, means 1&2&3&4.
when u put it at D with OD off, it will only uses the lower 3 gears, means 1&2&3 only.
when u put it to 2, regardless of OD on or off, it will only utilise the lower 2 gears, means 1&2 only.
when u put it to 1(if available), it will only use the lowest gear, means 1st gear only.

simple as that..

and the OD gear(4th or 5th gear) only comes in after certain speed, and depending on how u step on your pedal as well, if u floor it, it will not go into the OD gear..

for my dad's old ford telstar 2.0A, the OD will only comes in after 60kmh, and only if u press on the pedal lightly. if u step harder, it'll kick down to 3rd or 2nd gear to accelerate, and when u slowly lift the pedal, it'll eventually progress back to the OD gear.
so learn to control the pedal, and u'll be able to somehow "control" when u want the gear change to come in..