Grease a 9x13-inch pan; set aside.

Melt the butter in a large pot over medium-low heat. Add the marshmallows and stir until completely melted. Reduce the heat to low and stir in the Rice Krispies, then fold in the chopped Oreo cookies.

Turn the mixture into the prepared pan and press into an even layer. Allow to set at room temperature until firm, 30 minutes to 1 hour. Cut into squares and serve. Store leftovers at room temperature in an airtight container for up to 4 days.