Preheat your oven to 350°F (175°C) and lightly grease a small non-stick loaf pan with coconut oil or parchment paper.
In a large mixing bowl, mash the ripe banana with a fork until it reaches a smooth, liquid consistency.
Whisk in the whole egg, egg whites, Greek yogurt, and vanilla extract until the wet mixture is well combined.
Add the vanilla protein powder, oat flour, baking powder, cinnamon, and sea salt to the bowl, stirring gently until no dry clumps remain.
Fold in half of the chopped walnuts into the batter.
Pour the batter into the prepared loaf pan and level the top with a spatula.
Sprinkle the remaining chopped walnuts evenly over the top of the batter.
Bake for 30 to 35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow the loaf to cool in the pan for at least 10 minutes before slicing and serving.