Preheat the oven to 350℉. Spray muffin pans with cooking spray.
PREPARE THE TOPPING:
In a small bowl, stir together the oats, brown sugar, and cinnamon; set aside.
MAKE THE MUFFINS:
In a large bowl, use an electric mixer to beat the sugar and butter at medium speed until fluffy. Add the eggs, beating until blended. Stir in the vanilla.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon; add to the butter mixture alternately with the buttermilk, ending with the flour mixture. Gently stir in the blueberries. Spoon the batter into the prepared muffin pans, filling two-thirds full. Sprinkle the batter evenly with the oat topping mixture.
Bake 15 to 20 minutes or until the tops are golden. Cool the muffins in the pans for 5 minutes; remove from pans, and cool on wire racks.