As you know, bananas are packed with potassium and when they are overripe like those ones, you will get a lot of natural sugars that makes it a perfect whole food sweetener. You can add one or two, it is really up to you, or, alternatively, you can add puree dates or raisins as well. Blend the oatmeal very, very well (if you have oatmeal flour, you can use it instead) as this step will determine how smooth your chocolate pudding will look and feel. It's worth mentioning that it won't change the taste whatsoever, but the texture will differ depending on how fine your oatmeal flour is.