Preheat the oven to 425°F and place 18 muffin liners into two, 12-capacity muffin pan. Set aside.
In a mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt until well combined.
Using a hand mixer, beat together the butter and brown sugar in a large mixing bowl until light and fluffy.
Add the eggs and vanilla extract and continue beating until combined.
Set aside 2 tablespoons of the oats for the topping and stir in the remaining oats and Greek yogurt into the mixture (it will appear thick).
Add the mashed bananas and stir until combined.
Fold the dry ingredients into the banana mixture until just combined. Make sure not to over-mix!
Gently fold ¾ cup of the blueberries into the batter using a soft silicone spatula, making sure not to crush them in the process; otherwise, the batter will turn purple.
Let the batter rest for 10 minutes to help hydrate the oats.
Using a cookie scoop, divide the batter equally among the 17 muffin liners.
Sprinkle with demerara sugar, if desired, and top with the remaining blueberries and oats.
Bake for 5 minutes, then reduce the temperature to 350°F and continue baking for 12-15 minutes, or until a toothpick inserted into the muffin comes out clean.
Remove the muffin pan from the oven and place it on a wire rack to allow the muffins to cool for 10 minutes before removing them from the pan.