Add the rolled oats, skimmed milk powder and caster sugar, along with your choice of mix-ins, to a large bowl, and mix well. Transfer to an air-tight container to store for up to 6 weeks in a cool, dry area.
To serve, scoop about 70g (~ 1/2 cup) of the mixture into a bowl. Add 125ml (~1/2 cup) of boiling water (or however much you find you need to get to your desired consistency), and mix well. Leave for a few minutes, then eat with your choice of toppings.