Combine all the ingredients, stir thoroughly and place in a jar with a lid. Keep in the fridge overnight. Stir the mixture in the morning, add toasted hazelnuts, seeds and/or fruit and serve.
Use jumbo rolled oats, NOT instant
Substitutions: I used raisins but you could swap that for sultanas, which will add a nice citrusy touch to this recipe. You could also use dried cranberries, as well as chopped apricots. If you aren't keen on poppy seeds or can't get them use chia seeds instead.
As for the dairy, you could use any non-dairy milk and/or swap the yogurt for Skyr or quark or even a bit of ricotta cheese. These are all healthy, delicious substitutes.
Storing: Overnight oats can be refrigerated, covered, for up to 2 days.