I bought a 3rd gen 2 weeks ago, and I didn't get a manual with it.

My car is an automatic with 2, S and D.
I can only turn overdrive on and off when it's in S.
What's the point in having S where I can choose overdrive, and D where it's automatic?

What is S for?
The only thing I can think of, is "S" is short for sport, and changes gears at a higher RPM, and D is for economy. (Although this doesn't explain why I can't use the OD button in drive)

 

you pretty much hit the nail on the head. the s stands for sports shift. when you are in sports shift you have the choice of when to use 4th gear and when to take it out of 4th gear. the tranny will also shift a little bit higher than in drive. when in drive, the car goes through all the gears like it is suppose to. supposedly this was designed with the intensions of being able to hold third gear longer for "racing". kind of a pointless thing, but it sounds impressive when you say i have a sports shifter.