Combine flour, sugar, rolled oats, baking powder and salt in a large mixing bowl.
Beat your egg and mix together with mashed bananas and melted butter. Mash your bananas with a fork on a plate until they're completed mashed through. You don't want any large chunks of banana.
Add your wet ingredients to your dry ingredients. Add blueberries to the mixture. Begin mixing with a spatula gently until only JUST combined. Do not overmix. The batter should look lumpy.
Grease your muffin tin. Fill the muffin cups almost full.
Bake your muffins at 375 degrees F for 20-25 min, or until the top springs back when lightly touched. The top of the muffins should be medium-golden brown and crisp up in the oven.