Preheat oven to 375°F (190°C) and grease a standard 12-cup muffin tin or line with paper liners.
In a large mixing bowl, whisk together the rolled oats, whole wheat flour, baking powder, baking soda, salt, and ground cinnamon.
In a medium bowl, combine the mashed bananas, egg, maple syrup, Greek yogurt, milk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir with a spatula until just combined, ensuring no dry flour streaks remain.
Gently fold in the blueberries until evenly distributed throughout the batter.
Divide the batter equally among the 12 muffin cups, filling each nearly to the top.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.