after you change the oil. how do you get the change oil light to go out?
On my 2003 Silverado, you depressed the accelerator to the floor 3 times, then turned the ignition on. If that doesn't work, try it again with the ignition on, but the car not running. It might work, but if it doesn't work, call your local dealer, and they should tell you without a problem.
Check over on GAOwnersClub.com. My '02 has a reset button but I believe the earlier models used the key/gas pedal method.