Place all ingredients except the walnuts in a container and stir until well combined. Seal with a lid and place in the fridge overnight (or at least a couple hours).
Remove from the fridge in the morning and stir. You’ll notice that the oats soak up a lot of the liquid, so you may want to add a little more milk before serving.
Top oats with walnuts and any other toppings you like. Enjoy cold.
Overnight oats will keep for up to 5 days in the fridge in a sealed container.