In a high speed blender, blend the oats and water until totally combined - I usually do about 2 minutes.
Pour out the mixture into a nut milk bag (I got mine on amazon!) over a large bowl, and squeeze through the bag until there's only pulp in the bag and the liquid is in the bowl.
Use a funnel to pour the oat milk into a glass bottle to store in the refrigerator. The oat milk will separate quite a bit if it sits in the refrigerator for awhile, so make sure to shake well before using it.