Preheat oven to 180°C (350°F) and line a muffin tray with paper liners.
In a large bowl, mix oats, flour, baking powder, baking soda, cinnamon, and salt.
In another bowl, whisk together mashed banana, eggs, milk, oil, and vanilla extract until smooth.
Combine wet and dry ingredients and stir gently — avoid overmixing.
Fold in the grated apple.
Spoon batter evenly into muffin cups, filling about ¾ full.
Sprinkle a few oats on top before baking for a lightly toasted, bakery-style finish.
Bake for 18–22 minutes, or until golden and a toothpick comes out clean.
Let cool slightly before serving.