"A mild, sweet, oat drink popular in Panama and other Latin American countries. A childhood favorite of mine. You can leave this as is or, for creamier texture, blend with immersion blender before or after chilling."
Pour water into a tall pot; bring to a boil. Add oats; cook, stirring occasionally, in boiling water until a thin oatmeal forms, about 5 minutes. Remove pot from heat.
Stir cold water, condensed milk, and vanilla extract into oat mixture; transfer to a pitcher and chill in the refrigerator until cold, at least 1 hour. Pour drink into glasses and sprinkle cinnamon over the top.