Mix together flour, rolled oats, brown sugar and cinnamon.
Cut cold butter in using a pastry blender, two knives or just your fingers. The mixture should be completely mixed and crumbly. Set aside.
Peel apples, then core and slice into a large bowl. Sprinkle sugar, flour and cinnamon over the apples and gently toss to combine. Gently fold apple filling into prepared pie shell.
Pour the crumble mixture over the apples. Place pie pan on a cookie sheet. Lightly tent with foil.
Bake for 20 minutes, then reduce heat to 325°F and bake for another 50-60 minutes or until apples are soft when punctured with a fork.
Remove foil for the last 20 minutes of bake time. Allow to cool and rest before slicing for best results.
Notes
Cook and cool pie completely before freezing. Alternately, freeze before baking.
To bake from frozen, preheat oven to 400°F. Place pie on a baking sheet and bake for 30 minutes. Reduce heat to 350°F and continue baking for an additional 45 minutes or until apples are soft.