Spread 20 Oreos in the bottom of a 9x11 baking dish.
Top the Oreos with sweetened condensed milk and spread a layer of Cool Whip over the top.
Sprinkle the cake mix in the pan in an even layer and place the butter slices on top.
Bake in the preheated oven for 35 minutes.
Place the remaining Oreos in a bag and crush with a rolling pin. Sprinkle the crushed Oreos over the top of the cake and allow to cool slightly before serving.