Preheat the oven to 350°F (175°C). Line a 9 x 5 inch loaf pan with parchment paper.
Process the rolled oats in a high-powered blender until they form a fine flour, about 60 seconds.
Add the protein powder, sweetener (or brown/cane sugar), baking powder, cinnamon (if using), and sea salt to the oat flour and blend briefly until evenly combined.
In a large bowl, mash the ripe bananas until smooth (about 2 cups). Stir in the Greek yogurt, eggs, egg whites, avocado oil, and vanilla until well combined.
Pour the dry mixture into the wet ingredients and fold together until a thick, even batter forms.
Transfer the batter to the prepared loaf pan and smooth the top. Optionally sprinkle with chocolate chips if desired.
Cover the pan loosely with aluminum foil to allow room for rising and bake on the center rack for 40 minutes.
Remove the foil and continue baking another 10 to 20 minutes, until a skewer comes out clean or the internal temperature reaches 190–205°F (use a digital thermometer inserted into the center).
Let the loaf cool to room temperature in the pan before removing, slicing, and serving.
