Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, mix the rolled oats, Greek yogurt, almond milk, honey (or maple syrup), and applesauce until well combined.
In another bowl, whisk together the protein powder, baking powder, baking soda, cinnamon, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Gently fold in the mixed berries.
Pour the batter into the prepared muffin tin, filling each cup about 3/4 full. Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.