Preheat oven to 500F and line or grease a standard-size muffin tin.
In a large bowl, whisk together the oats, oat flour, all-purpose flour, baking powder, baking soda, and salt.
In a separate bowl, rapidly beat together the brown sugar, buttermilk, vegetable oil, egg and vanilla, for about 30 seconds.
Fold the liquid ingredients into the dry until just combined. Fold in chopped rhubarb.
Make the Crumb Topping
Mix together the butter, rolled oats, brown sugar, and cinnamon for the topping until moist crumbs form.
Finish the Muffins
Divide batter evenly among prepared muffin cups. Top muffins with equal amounts of crumb topping. Place muffins in oven and adjust temperature to 400F. Bake for 15-17 minutes, until a toothpick inserted into the center of one of the muffins comes out clean. Allow muffins to cool slightly in tin before transferring to a wire rack to cool completely.