Preheat the oven to 350°F (175°C) and line a muffin tin with liners.
Grind the rolled oats into flour using a blender or food processor.
Mash the bananas in a large bowl, then whisk in the eggs and yogurt.
Stir in the oat flour and baking powder (and cinnamon or vanilla, if using) until just combined.
Divide the batter evenly among the muffin cups.
Bake for 15-18 minutes, until a toothpick inserted in the center comes out clean.
Cool on a rack before serving.