Crush the Oreos into fine crumbs and mix with melted butter.
Press the mixture into a 9x13-inch pan and refrigerate to set.
In a bowl, beat cream cheese until fluffy. Add sugar and 2 tablespoons milk; mix well.
Fold in 1 1/4 cups Cool Whip and spread over the crust. Refrigerate.
Whisk together pudding mix and 2 3/4 cups cold milk until thick. Spread over the cream cheese layer. Refrigerate until firm.
Top with mini marshmallows, then spread 1 1/2 cups Cool Whip on top.
Sprinkle with mini chocolate chips. Chill for at least 4 hours or overnight before serving.