Pour 1 cup old fashioned rolled oats into a medium bowl, and pour enough cold water over them to cover the oats by at least 2 inches. Let the oats soak for 10 minutes. Avoid over-soaking.
Drain the oats through a sieve or colander, and rinse with cold water. Transfer the rinsed oats to a blender. Add 1 Tablespoon sugar, a pinch kosher salt (if using), and 4 cups cold water.
Blend the oats for about 10-15 seconds. Strain the mixture through a very fine strainer (eg. a reusable coffee filter or yogurt strainer) or a fine-mesh strainer lined with 3 layers of cheesecloth or a clean flour sack towel. If you still see any sediment in the milk, rinse the strainer and/or cloths and strain again.
Store the milk in an airtight jar or bottle in the fridge, shaking before use. Use within 2 days.