Heat a large saucepan over medium low heat. Add butter and let it melt.
Grease a 13x9 inch pan with non-stick spray. Set aside.
Once the butter has melted add the marshmallows and stir them until they have fully melted. Approximately 5-7 minutes.
When the marshmallows have fully melted add the Rice Krispies and then crushed Oreos.
Stir together the Rice Krispies, Oreos, and marshmallows until the marshmallow coats everything.
Lightly coat your hands in a little bit of butter or non-stick spray and then, while the mixture is warm, pour it into the greased pan and use a spatula and your hands to press it down into the pan making a solid layer of Oreo rice krispie treats.