Blown head gasket.
double check with a service manual, but you should be able to just drain it unbolt it replace the gasket and bolt on a new one
undo bolts , take off old one put on new one replace bolts tourque to 106.
Replacing the head gasket on any vehicle is a long and complicated process, however if you wish to see basic step-by-step instructions go to the related question on the right side of the page "How do you replace a head gasket?"
replace head gasket
blown head gasket, cracked cylinder head, crack in the cylinder block
To replace the oil pan gasket on the Cavalier you need the tool box to open where it is located. After opening, you can proceed to replace the oil pan gasket.
If the vehicle isn't overheating then it's probably not a blown head gasket. There could be a problem with the oxygen censor.
Probably a head gasket problem or a cracked head. Sorry for the bad news.
Probably a blown head gasket. If the car has an (engine) oil cooler it might be a leak there too.
My 1998 Breeze did that same thing within the first two years I had it. It ended up being the head gasket needing replaced.
remove old thermostat and replace with new thermostat and gasket