Line a baking sheet with waxed paper. Set aside.
Add oil and popcorn kernels in an even layer to a 4-quart saucepan with a heavy bottom.
Cover pan with a lid left slightly ajar and place over medium heat.
Popcorn will pop after a few minutes. When popping slows to a few seconds between pops, remove pan from heat and transfer popcorn to a large bowl.
Using a spoon so you don't burn your fingers, remove and discard any unpopped kernels. Set aside.
Melt butter in a medium saucepan over low heat.
Add mini marshmallows and sugar, stirring constantly until completely melted.
Pour marshmallow mixture over popcorn then stir until evenly coated.
Coat hands with nonstick cooking spray then, working quickly while the melted marshmallows are still warm, form popcorn into baseball-sized balls.
Roll popcorn balls in Halloween sprinkles then add candy eyeballs.
Place popcorn balls on your baking sheet until cooled and set then enjoy.
Cover leftovers tightly with plastic wrap and store at room temperature up to 3 days.