Cut the banana into small pieces and fill with the oat flakes, vanilla and cranberries in a large screw-top jar, e.g. a tumbler with 420 ml capacity. Add the buttermilk and mix everything well and place in the refrigerator overnight. Stir again the next morning and enjoy.
Notes: You may need a little more buttermilk. That always depends on the oatmeal used. I don`t use any additional sweetener for the overnight oats because the cranberries and banana are sweet enough for me. If you like it sweeter, you should sweeten the overnight oats to taste.