Preheat your oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a bowl, combine the oats and milk. Let them soak for about 10 minutes.
In another bowl, whisk together the egg, applesauce, oil, brown sugar, and vanilla extract.
In a separate bowl, mix the flour, baking powder, baking soda, salt, and cinnamon.
Add the wet ingredients to the soaked oats and milk mixture. Stir to combine.
Gradually add the dry ingredients to the wet mixture. Fold in the diced apple.
Divide the batter evenly among the muffin cups.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes, then transfer to a wire rack to cool completely.