Preheat your oven to 350°F (175°C). Grease the muffin tin or line with paper liners.
In a large mixing bowl, combine the rolled oats and milk. Let it sit for about 5 minutes until the oats soften.
In a separate bowl, mix the mashed banana, Greek yogurt, honey or maple syrup, vegetable oil, and vanilla extract until well combined.
Add the banana mixture to the oats and stir thoroughly.
In another bowl, whisk together the baking powder, baking soda, salt, and ground cinnamon. Then fold this mixture into the wet ingredients until just combined. Do not overmix.
Gently fold in the mixed berries and chopped nuts until evenly distributed.
Evenly distribute the muffin batter into the prepared muffin tin. Fill each cup about ¾ full.
Bake in the preheated oven for 20 minutes or until the muffins are golden brown and a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely.