Prepare all ingredients, mash bananas. Whisk eggs together with buttermilk and vanilla extract. Whisk mashed bananas into the mixture.
Combine the dry ingredients (flour, oats, sugar, baking powder and baking soda) all together. In small parts ,mix the dry ingredients into the liquid mixture.
Stir dry cranberries into the muffin batter. Line muffin pan with paper liners, fill each one about 3/4 to the top. I had about 18 muffins.
In a separate bowl, combine oats with brown sugar. Sprinkle muffin tops with the mixture. Bake muffins at 350F for about 20-30 minutes. Insert a skewer into the center of a muffin, to test for readiness; when it comes out clean, the muffins are ready.
Let muffins cool to a room temperature.