Spray a 9x13 casserole dish with cooking spray (or spread melted butter all over the bottom and sides).

In a large resealable bag, add the Oreo cookies and use a rolling pin to crush them up (you want a mix of fine pieces and chunks).

Add the Rice Krispies cereal and crushed Oreos to a large mixing bowl and stir together.

In a saucepan over medium heat, melt the butter. Add the marshmallows and stir until the marshmallows are completely melted.

Pour the marshmallow mixture over the Rice Krispies and use a spatula to stir until everything is completely covered with marshmallow.

Pour the mixture into the casserole dish and press down firmly (you can use the spatula for this, but I prefer using a gloved hand).

Allow to cool completely before slicing.