Preheat oven to 350°F (175°C). Grease a muffin pan and set aside.
In a small bowl, mix ground flaxseed with water and let thicken for 5 minutes.
In a large bowl, combine oats, baking powder, salt, and cinnamon.
In another bowl, whisk mashed banana, flax mixture, melted coconut oil, almond milk, maple syrup, and vanilla.
Add wet ingredients to dry and mix well. Gently fold in blueberries.
Divide batter evenly into muffin cups.
Bake for 23–25 minutes until golden and set.
Cool 5 minutes, then remove and serve warm.