Preheat oven to 350 F. Combine graham cracker crumbs, 1 cup Oreo cookie crumbs and butter and press into a 9-inch pie tin; bake 8 minutes. Set aside to cool. In double boiler or in bowl set over a pan of hot water, melt chocolate with 2 cups cream, stirring frequently. Remove from heat and stir until cool. Whip remaining cream with sugar until stiff; fold into cooled chocolate with remaining crushed Oreos. Pour into crust and refrigerate until well chilled and firm. Garnish with whipped cream and chocolate shavings.