Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, combine the rolled oats, mashed bananas, Greek yogurt, honey (or maple syrup), brown sugar, eggs, and vanilla extract. Mix until well combined.
In a separate bowl, whisk together the baking powder, baking soda, salt, and ground cinnamon.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. If using, fold in the chopped walnuts or chocolate chips.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.