Preheat oven to 375. In a large bowl combine apples and sugar. Transfer to 9x13" dish, set aside.
For topping, in a medium bowl combine the oats, brown sugar, flour, and cinnamon. Cut in butter until mixture resembles coarse crumbs. Sprinkle topping over apple mixture.
Bake for 35 to 40 minutes or until apples are tender and topping is golden. If desired, serve warm with ice cream.