Preheat the oven to 350°F (177°C). Spray a 12-count muffin pan generously with non-stick cooking spray and set aside.
In a large mixing bowl, combine the rolled oats, baking powder, ground cinnamon, and salt.
In a separate bowl, whisk together the milk, eggs, maple syrup, and vanilla extract until smooth and fully combined.
Pour the wet ingredients into the dry ingredients and stir until everything is evenly incorporated.
Gently fold in the chopped apple.
Divide the mixture evenly among the 12 prepared muffin cavities.
Bake the Apple Cinnamon Baked Oatmeal Cups for 25 to 27 minutes, or until the tops are lightly golden and the oatmeal cups feel firm.
Remove the pan from the oven and let the cups cool for 5 to 10 minutes.
Carefully remove the oatmeal cups and transfer them to a wire rack to finish cooling.