Gather all your ingredients. Mash the ripe bananas. Whisk eggs with buttermilk and vanilla extract. Fold mashed bananas into the mixture.
Combine the dry ingredients (flour, oats, sugar, baking powder and baking soda) together. In small parts, mix the dry ingredients into the wet batter.
Stir in dry cranberries.
Line muffin pan with paper liners and fill each one about 3/4 of the way to the top.
Muffin Topping
In a separate bowl, combine oats with brown sugar. Sprinkle equal parts of the mixture on top of each muffin.
Bake muffins at 350°F for about 20-30 minutes. Insert a skewer into the center of a muffin to test for doneness. If it comes out clean, the muffins are ready.