Combine oats, yogurt, milk, flax, salt and cinnamon in a glass jar or small bowl and stir well. Seal jar with lid or bowl with plastic wrap and refrigerate overnight.
In the morning, stir in banana slices, peanut butter and berries to serve.
Notes
Steel cuts will have a nice chew to them but if you prefer to cook them further, add 1/4 cup extra milk and microwave for 90 seconds.