Preheat your oven to 375°F (190°C) and lightly grease a baking dish with melted butter.
In a large mixing bowl, combine the oats, cinnamon, salt, and brown sugar, whisking everything together until evenly dispersed.
Pour the milk into the dry mix, add vanilla extract, and stir until the mixture is well combined and slightly thickened.
Gently fold in the sliced apples, distributing them evenly throughout the oatmeal mixture.
Pour the mixture into the prepared baking dish, spreading it out into an even layer with a spatula.
Sprinkle the top with a little extra cinnamon and drizzle with melted butter for a golden, flavorful crust.
Bake uncovered for about 30-35 minutes, until the top is bubbling and golden brown.
Remove from the oven and let cool slightly before serving. The oatmeal should be soft, with tender apple slices and a fragrant cinnamon aroma.