Preheat the oven to 200oC/Gas Mark 6.
Mash the bananas in a bowl, then add the oats, cinnamon and water and mix together.
Line a square baking tin with baking paper, then pour in the mix. Press down firmly with the back of a fork to compact it into the tin.
Bake in the oven for 20-25 minutes, remove from the oven, then allow to cool for 10 minutes before slicing into bars.