Crush Oreos in a food processor to a fine crumb consistency. Add melted butter and mix well. Press mixture into bottom of a 10-inch pie pan; freeze.
Melt chocolate, sugar, butter and milk in the top of a double boiler. Cook over boiling water until sauce thickens slightly, stirring continuously; cool.
Spread softened ice cream carefully onto crust. Return to freezer. Whip cream with liqueur until stiff, but not dry. Pour cooled chocolate sauce evenly over ice cream, then spread whipped cream topping over chocolate sauce. Cover with plastic wrap and freeze 4 hours or overnight. Remove pie from freezer 20 minutes before serving.