Soak 1 cup rolled oats in cold water overnight (or warm water for 20min).
Rinse well, in a sieve.
Add to a blender with 3 cups fresh water.
Blend for about 1 minute.
Strain through a nut milk bag.
Optional: put it back in the (rinsed) blender, and salt, vanilla and sweetener, and blend.
Bottle and refrigerate, and use within one week.