Soak oat groats 6-8 hours or overnight in filtered water and apple cider vinegar.
Drain and rinse the soaked groats, add to a blender.
Add the 4 cups of fresh water to the blender, blend on high for 30 seconds.
Pour through a nut-milk bag and squeeze out the liquid, reserving the pulp for porridge, baking, etc.
Pour the milk into a clean container and store in the fridge for 4-5 days. Shake before using as it will separate! Freeze any leftovers in ice cube trays.