Preheat the oven to 400 degrees F. Lightly grease a standard 12-cup muffin tin or line with paper liners and set aside.
In a large bowl, stir together the whole wheat flour, oats, oat flour, baking powder, cinnamon, salt, and nutmeg.
In a separate bowl, whisk together the milk, maple syrup, oil (or butter), eggs, and vanilla. Make a well in the center of the dry ingredients, then pour the wet ingredients into the center.
By hand, stir the batter gently, just until combined (it will be somewhat lumpy). Divide the batter evenly between the prepared muffin cups.
For the crumb topping: In a small bowl, quickly rub the butter, flour, brown sugar, and cinnamon together with your fingers until fine crumbs form. Sprinkle over each unbaked muffin.
Bake the muffins until golden brown and a toothpick in the center comes out clean, 18-20 minutes. Allow the muffins to cool in the pan for 5 minutes, then gently remove to a wire rack to cool completely.
Store leftover muffins in an airtight container at room temperature for up to two days or individually wrap and freeze for up to two months.