Preheat the oven to 350°F (175°C) and lightly grease a baking dish.
Wash and core the apples, leaving the bottom intact to hold the filling.
In a bowl, mix rolled oats, brown sugar, cinnamon, and butter until crumbly.
Stuff the oat mixture into the hollowed apples, pressing gently.
Place the apples in the baking dish and add a few tablespoons of water to the bottom.
Bake for 30 minutes or until the apples are soft and the topping is golden.
Let them cool slightly before serving with ice cream or yogurt.
Enjoy warm for the best flavor!