Grind the oats into a powder, using either a food processor or blender. Alternatively, a mortar and pestle makes a terrific grinder for a small amount.
Pour water into a small saucepan. Bring to a rolling boil. Add the powdered oats to the boiling water, stirring constantly for about 30 seconds.
Cover the pot, turn down the heat to low, and simmer for 8–10 minutes, or until the oats are smooth and thick. Stir occasionally to prevent sticking and burning.
Peel the banana, removing any brown spots. Fork-mash on a plate until completely creamed.
Mix the banana with the cooled cereal. Add breastmilk or formula to thin, if necessary.