The oil change light is on steadily despite changing of oil a couple of weeks ago

The red oil warning light is for low oil pressure. Not necessarily low oil level in the engine.

You probably have a blowin head gasket, Those car are Knowin for that (Problem)

it wont go away because their is a mechanical problem and or problem with oil change [ not enough oil wrong filter etc ] will need to have vehicle scanned for the fault code to determine the problem most auto stores offer this service for free

Go to a trust worthy mechanic. They should be able to clean the undercarriage, and then add a UV fluorescent dye to your oil. After a couple of weeks, you go back and they can check the underside with a black light to find the leak.

The oil system reminder needs to be reset so the computer knows you had the oil changed. The procedure is in the owners manual but can be tough to find so you might have to do a little searching for it. Many of the GM vehicles can be reset by turning the ignition key to "run" [but don't crank or start the engine] and then fully depressing and releasing the accellerator pedal smothly three times within 5 seconds. Turn the key off, wait about 10 seconds and restart the engine to see if the reset was successful. Check your manual first as not all GM's can be reset this way and yours may be different.

disconnect the +battery post with an 8mm socket and leave the thing off for 2 minutes.reconnect it.

I belive they didn't reset the oil light timer. I belive the sequence is 1. key on (not acc with engine off)
2. step on the brake pedal 5 times within 10 seconds. 3. key off. 4. start car. 5. check if the light went out. you may have to try it a few times to get it.

Check your owners book, its a comination push of reset and average buttons on the fuel display.

Could be a number of things. Should still be under 3/36000 warranty. Take it to the dealership. If the place you got your oil change from used the wrong weight oil, this could cause the light to come on. There are thousands of possibilities, which is why you need a computer to check them now.

First thing I would do is replace the oil pressure sending unit for the gauge.