Peel, core and slice apples.

Preheat oven to 375.

Layer apples into baking dish (I used 10x7.5").

Mix together flour, brown sugar, oats and cinnamon in a bowl.

Add in vanilla and cut in butter with a pastry blender or fork.

Work until mixture forms large crumbs.

Sprinkle over apples.

Bake 30-35 minutes or until topping is browned and apples are soft.