Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the oat flour, vanilla whey protein powder, baking powder, baking soda, and salt until well combined.
In a separate medium bowl, beat the eggs, then add the Greek yogurt, unsweetened applesauce, almond milk, and vanilla extract. Mix until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Do not overmix.
Fold in the chopped walnuts evenly throughout the batter.
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.