In a high speed blender, add oats and 5 cups of water. Blend for 30 seconds.
Pour oat mixture through a nut milk bag, or fine mesh strainer. Do not squeeze the bag, or your milk will become gummy from squeezing the oats. You can lightely squeeze if you're in a hurry, but try to avoid it.
Hang your nut milk bag on a cabinet door, with the bowl underneath, and let it just do it's thing for a few minutes.
Now add the additional water, sweetener, vanilla extract, and salt. Stir together to combine.
Pour oat milk into a jar and store in the refrigerator for up to 3 days. The oat milk will separate. Simply shake the milk before using.