Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with butter or cooking spray.
Core and dice the apples into small, bite-sized pieces. Peel them first if you prefer a softer texture.
Add the rolled oats, cinnamon, baking powder, and salt to a large mixing bowl. Stir until evenly combined.
In a separate bowl, whisk together the eggs, milk, maple syrup, melted butter, and vanilla extract.
Pour the wet ingredients over the oat mixture and stir until all the oats are evenly coated.
Gently fold in the diced apples, distributing them throughout the mixture.
Transfer the Apple and Cinnamon Baked Oatmeal mixture to the prepared baking dish and spread it into an even layer.
Bake uncovered for 40–45 minutes, or until the center is set and the top is lightly golden.
Allow the oatmeal to cool for about 10 minutes before slicing and serving.