For topping, combine walnuts, 1/3 cup flour, oats, 1/4 cup sugar, 1 teaspoon cinnamon and butter until crumbly. Pour filling into prepared crust. Sprinkle with topping. Bake 35-40 minutes or until filling is bubbly and streusel is crisp. (If needed, cover pie loosely with aluminum foil during last 10 minutes to avoid overbrowning). Cool pie in pan on cooling rack at least 1 hour before serving.