most of the time its through the radio. turn the radio off, and then hold tune until you get a settings message. Then using the seek button scroll through the menu. You should see an OIL LIFE message. Then hit the button next to seek, it should say reset, and you hit tune. It'll then say DONE.
Just to elaborate on how it works a bit more than capricewiz did.
It in no way monitors oil condition.
It considers operating temps, cold starts and use.
I have not seen any solid explaination but some have guessed that it looks at cold starts and counts crankshaft revolutions. Stop and go has the crank turning a LOT for the number of miles driven, driving with your foor to the floor has the crank turning a lot too so it seems to be a sound theory but that is all it is far as I know.
I was confused by exactly this a few months after I bought my sedan, it was the first car I ever owned with a change oil light.