In a lidded container, stir the milk, yogurt, peanut butter, and cinnamon until mostly smooth.
Mix in the oats, cover, and refrigerate overnight.
In the morning, stir again. Add a splash of milk if the oats are thicker than you like.
Top with banana just before eating. Taste before adding optional syrup.
Storage: For two breakfasts, double the mixture and divide between clean containers. Refrigerate promptly and keep cold; use within 3–4 days, or sooner if an ingredient’s use-by guidance requires it. Add sliced fruit at serving time for the best texture. Carry in an insulated bag with a cold pack.