Preheat the Oven (5 minutes): Preheat your oven to 350°F (175°C). Line a 12 cup muffin tin with paper liners or lightly grease with cooking spray.
Prepare the Dry Ingredients (5 minutes): In a large bowl, combine rolled oats, flour, baking powder, baking soda, cinnamon, and salt. Mix well.
Mix the Wet Ingredients (5 minutes): In another bowl, whisk together mashed bananas, brown sugar, Greek yogurt, milk, vegetable oil, eggs, and vanilla extract until smooth.
Combine Ingredients (5 minutes): Gradually add the dry ingredients to the wet ingredients. Stir until just combined. Avoid overmixing. Fold in chocolate chips and walnuts if using.
Fill the Muffin Tin (5 minutes): Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake the Muffins (20 minutes): Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Cool and Serve (10 minutes): Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Serve warm or at room temperature.