Preheat oven to 350°F (175°C) and grease a 9x5 inch non-stick loaf pan.
In a large mixing bowl, whisk together mashed bananas, eggs, egg whites, Greek yogurt, and vanilla extract until homogenous.
In a separate container, combine whey protein isolate, oat flour, baking soda, baking powder, salt, and cinnamon.
Gradually fold the dry ingredients into the wet banana mixture using a spatula until just combined, ensuring no large dry pockets remain.
Pour batter into the prepared loaf pan and level the surface with a spatula.
Bake for 45 to 50 minutes or until the internal temperature reaches 200°F (93°C) and a skewer comes out clean.
Remove from oven and let rest in the pan for 10 minutes.
Transfer to a wire rack to cool completely before slicing into 10 even portions.