Peel and thinly slice the apples, then arrange them evenly in a prepared baking dish.

Add the brown sugar and softened butter to a mixing bowl and combine until well incorporated.

Add the all-purpose flour, oats, and ground cinnamon to the butter mixture.

Mix until the ingredients form a crumbly streusel topping.

Evenly layer the streusel mixture over the sliced apples, covering the surface.

Bake at 350°F (175°C) for 30–35 minutes, or until the apples are tender and the topping is golden.

Remove from the oven and allow it to cool slightly before serving.

Serve warm and enjoy.