2002 Oldsmobile Bravada low idle and stall when AC is on

Car has 106,500 miles V6 AWD, the day I purchase a new battery the RPM went to 400-600. We switched it back to the old battery and the same thing happen, car will turn off. I could not get a reading, it could not read any code. I though maybe it was a bad MAP sensor and so I bought a brand new MAP sensor but it made it worse. It won’t even stay on without having to throttle the gas pedal. I switch the MAP sensor back to the old one and the car stays on, but at 400-600. The second I turn on the AC, the idle drops to zero and car shuts off. The STOP ENGINE light will come on and at times it will say LOW OIL PRESSURE ( the gauge will drop) I checked the oil and its good. It was low so I added more. I checked if there was enough gas coming through from the bottom and reading were good. I read that maybe it could be a BAR sensor but I don’t see it on the car. I was told to replace the EGR valve, but can’t find it anywhere in the car ( where is it??) The car will start and run smoothly at 400-600 RPM but AC will cut it off. Please help. Tuco…summer is coming its going to b a hot one this year

This car may have an idle up AC actuator, if it does that may be the culprit. It is supposed to increase the idle with the additional load of the AC.

I am confused by your comment on the oil 'I checked the oil and its good. It was low so I added more." The oil could be good, bad or ugly

Yeah there is a switch there or something but would need the service manual to identify what or where. It also may be because you disconnected the battery, the computer lost it’s memory and will take a while to relearn again.

Pull the throttle body , thoroughly clean both sides of the plate and interior surface and reset the pcm . Use only throttle body cleaner , do not use like carb cleaner .Pull the fuse that feeds it for like 10-15 minutes . See if that helps . If you replaced the battery then the pcm now thinks the throttle body is clean but it is not so you have to clean it . Eventually it would relearn the dirty throttle body parameters and could possibly straighten itself out but it may take awhile .

Sounds like possibly an Idle Air Control valve issue (among others) but you need to clarify a few things.
You say the idle speed is 400 to 600. Which is it? 400 is too low and 600 is about right. Are you saying the idle is fluctuating?
Your comment about the oil also raises a question. How much oil did you have to add?