Line twelve 2 1/2-inch muffin cups with paper bake cups.
Place 2 cups rolled oats in a food processor bowl or blender container. Cover and process/blend until fine.
Transfer oats to a large bowl; stir in whole wheat flour, sugar, baking powder, 3/4 teaspoon cinnamon (or apple pie spice), baking soda, and salt. Make a well in the center of the flour mixture and set aside.
In a medium bowl, whisk together buttermilk, banana, eggs, 2 tablespoons butter, and vanilla.
Add buttermilk mixture all at once to well of flour mixture, stir just until moistened. Spoon batter into muffin cups.
Stir together 1/4 cup oats and 1/2 teaspoon apple pie spice. Cut in 1 tablespoon butter, until mixture is crumbly.
Sprinkle oat mixture atop the muffin batter.
Bake in 350 degree oven for 20-22 minutes, or until a toothpick inserted in center of muffin comes out clean.