Preheat oven 400ºF. Fit a 9-in. pie pan with pie dough, fluting edges of dough.
In small bowl, combine 1/2 cup sugar, 2 tablespoons flour and 2 teaspoons cinnamon. Sprinkle sugar mixture over apple slices in large bowl; mix well to coat.
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.