Combine flour, sugar, and salt; cut in shortening with pastry blender until mix resembles coarse meal. Stir in oats. Sprinkle cold water (1 tablespoon at a time) evenly over surface of mix; stir with a fork until dry ingredients are thoroughly moistened. Shape mix into a ball; chill.
Roll dough on a lightly floured surface to fit a 9 inch pie plate. Place in pie plate; trim off excess pastry along edges. Fold edges under and flute. Prick bottom and sides of pastry generously with a fork. Bake at 425°F for 10-12 minutes.
