Preheat the oven to 350 F. Grease a 9-x9-inch casserole baking dish with butter or oil spray. Core, peel, and slice the apples into ½-inch thick half-moons. Arrange the apple slices in the bottom of the prepared casserole dish.
In a large bowl, mix together the rolled oats, brown sugar, and cinnamon.
Microwave the butter in 20-second intervals, until it is fully melted, about 40 seconds. Stir the butter into oat mixture, and pour it over the apple slices.
Bake until the oat topping is golden brown, and the apple slices have softened, about 35 minutes. Serve warm, or let cool completely before serving.