Preheat oven to 375°F. Spray a 12 cup standard size muffin tin with baking spray. Set aside. Alternately, you may use cupcake liners.
In a medium-size mixing bowl, use a whisk to sift together flour, oats, brown sugar, baking powder, cinnamon and salt.
In a separate bowl, whisk together buttermilk, butter, eggs and vanilla. Mix wet ingredients into the dry ingredients until fully moistened. Mix well.
Divide batter evenly between muffin cups.
To make the streusel: Mix together oats, flour, sugar and cinnamon. Use a pastry blender or 2 knives to cut in cold butter until it resembles crumbs. Sprinkle evenly on top of muffins.
Bake for 18-20 minutes or until a toothpick inserted into the center comes back clean.
Cool for 10 minutes in the pan, then remove to a cooling rack or platter. Serve warm.