Preheat oven to 375°F (190°C) and grease a 12-count muffin tin or line with paper liners.
In a large mixing bowl, whisk together the whole wheat flour, rolled oats, baking powder, baking soda, salt, and cinnamon.
In a medium bowl, combine the mashed bananas, egg, honey, melted coconut oil, milk, and vanilla extract until well blended.
Pour the wet ingredients into the dry ingredients and stir with a spatula until just combined; do not overmix.
Spoon the batter evenly into the prepared muffin tin, filling each cup about three-quarters full.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.