2002 Pontiac Grand Am

There are 2 likely hings that can cause this, and 2 unlikely but possible.

The likely things are a blownheadgasket/warped head, and a bad manifold gasket if it is a wet type manifold.

The two unlikely thigns are is the block is cracked or someone dumped oil in the antifreeze or vice versa.

Best thing to do is buy a compression tester and check the compression to see if its head/headgasket related or no.