In a milk pan, melt marshmallows in the milk over low heat. Let cool it slightly.
Make the tart crust. Crush the Oreo with a food processor, mix in the melted butter, and put into a 6-inch pie plate. Press down with your hand. Keep in the refrigerate for about 30 min.
In a bowl, add the sugar to the heavy cream and whip it. Fold the marshmallow mixture into the whipped cream. Then add the crushed Oreo cookies.
Pour the cream filling into the pie plate. Let it cool in the refrigerate at least 6 hours.
Decorate with more crushed Oreos.