First, gather all your ingredients. Preheat your oven to 350°F (175°C) to prepare for baking.
Begin by coring the apples, making sure to create a large cavity in the center of each. Arrange the apples on a rimmed baking sheet.
In a mixing bowl, combine the rolled oats, brown sugar, and cinnamon. Add the cubed butter, and using a fork or your hands, mix until the butter is fully incorporated and the mixture is crumbly.
Fill each apple with the oat mixture, distributing it evenly. Place the baking sheet in the oven and bake for around 30 minutes, or until the apples are tender and the filling is bubbling.
Let the baked apples cool slightly before serving, allowing the flavors to settle.