In a blender, combine the water, oats, maple syrup, vanilla extract, and sea salt. Blend on high speed for about 30 seconds until the mixture becomes smooth and well-combined.
Place a fine-mesh strainer over a large bowl and pour the blended mixture through the strainer. Allow the liquid to pass through naturally, avoiding the temptation to press the pulp, as this will keep the oat milk smooth and free of grittiness.
Taste the milk and adjust the sweetness by adding more maple syrup if desired. For the best flavor, refrigerate the oat milk overnight to chill thoroughly. If serving immediately, pour over ice to enjoy it at its peak freshness.