Preheat the oven to 350 degrees and lightly grease an 8-inch baking dish.
Place the sliced apples in a mixing bowl. Add the granulated sugar and cinnamon, then stir until the apples are evenly coated.
Spread the apple mixture into the prepared baking dish in an even layer.
In the same mixing bowl, combine the rolled oats, flour, and brown sugar. Pour the melted butter over the mixture and stir until it forms a crumbly topping.
Sprinkle the oat topping evenly over the apples.
Bake for 35 to 40 minutes until the topping turns golden and the apples feel tender when pierced with a fork.
Remove the dish from the oven and let it rest for 5 minutes. Serve warm with scoops of vanilla ice cream.