Arrange apples, pared, cored, and thinly sliced in a well-buttered shallow baking dish. Melt the butter and stir in the sugar, oatmeal, flour, and cinnamon until well mixed. Sprinkle over the apples. Bake in a moderate oven (350 degrees F) for 45 minutes until the crust is golden brown and the apples are soft. Serve the crisp warm with whipped cream, vanilla ice cream, or cheddar cheese.