Preheat oven, line muffin tin, and mix dry ingredients: Preheat oven to 350°F. Line a standard 12-cup muffin tin with baking cups. In a medium bowl, whisk together flour, oats, baking powder, salt, and cinnamon.
Combine wet ingredients: In a separate bowl, whisk together sugar, eggs, butter, and milk.
Add wet ingredients to dry: Stir wet ingredients into dry ingredients, mixing just until combined.
Add apple: Stir apple into batter.
Divide batter between muffin cups: Divide batter evenly between muffin cups, adding a scant ¼ cup to each. Sprinkle with crumb topping if desired.
Bake and cool: Bake until tops spring back when lightly touched, about 20 minutes. Cool in pans 5 minutes, then remove to wire rack to cool completely.
