Preheat the oven to 350°F (175°C) and line a muffin pan with paper liners.
In a large bowl, mash the ripe bananas.
Mix in the Greek yogurt, egg, vanilla extract, and sweetener until smooth.
Add rolled oats, oat flour, baking powder, and baking soda. Stir until combined.
Fold in chocolate chips if desired.
Pour the batter into the muffin pan, filling each cup about 3/4 full.
Bake for 18-20 minutes or until a toothpick inserted comes out clean.
Let muffins cool for 5 minutes in the pan, then transfer to a wire rack.