Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease the cups.
In a large bowl, combine the rolled oats, whole wheat flour, baking powder, baking soda, ground cinnamon, and salt. Stir until evenly mixed.
In a separate medium bowl, whisk together the eggs, Greek yogurt, honey, unsweetened applesauce, milk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix.
Fold in the fresh blueberries and chopped walnuts carefully to distribute evenly.
Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
Once cooled, store the muffins in an airtight container for up to 3 days at room temperature or up to 1 week refrigerated.