Add all of the ingredients except the hazelnuts to a jar or bowl and stir to combine until the Nutella and cocoa powder are completely mixed in.
Cover and refrigerate overnight or a minimum of four hours.
Remove from the fridge, stir the oatmeal, and serve cold or hot. Top with toasted hazelnuts (optional), more Nutella, and any other desired toppings. To heat, microwave in 30-second intervals.
Video
Notes
Topping ideas: strawberries, bananas, chopped toasted hazelnuts, Nutella, coconut flakes, granola.Storage: Refrigerate in an airtight container for up to 5 days. The old-fashioned oats will become softer over time.