In a small jar, layer oats and chia seeds. Seal jar and store in a dry place up to 6 months.
When ready to prepare, fill the jar almost to the top with the milk of your choice. Use more milk to make a runnier oatmeal, or use less milk to allow it to firm up more.
Stir in peanut butter, maple syrup (or the sweetener of your choice) and cinnamon. Seal and shake jar.
Leave overnight in the fridge to rest for at least 8 hours, up to 4 days. Enjoy cold.