Mix popcorn, M&Ms, and peanuts in a large bowl.

Heat oil, butter, and marshmallows in a pan over medium-low heat until melted. Pour over popcorn and blend together with a heavy spoon.

Spray Bundt cake pan with nonstick cooking spray; press popcorn mixture lightly into the pan and refrigerate until cool.

To remove popcorn cake from the pan, place the pan in warm water, then turn it upside down until cake comes out.