Two things are going to help make these muffins moist and fluffy. First is the ripe bananas – make sure to use bananas with brown spots and not green ones. These will be sweeter and add to the moisture. The second is letting the oats have time to soak up wet ingredients before baking. This is why I let the muffin batter to sit for 15-20 minutes before adding it to the muffin pan.