Prep: Grease a 9×13-inch baking dish. Preheat oven to 325°F.
Mix Dry Ingredients: Whisk oats, baking powder, cinnamon, nutmeg, salt, and brown sugar in a large bowl.
Combine Wet Ingredients: In a separate bowl, whisk milk, eggs, and vanilla.
Assemble: Pour wet ingredients into dry. Stir in melted butter.
Layer: Scatter apples in the dish. Pour oatmeal mixture over top.
Bake: 35-45 mins until golden and set. Cool slightly before serving.