Preheat a waffle iron according to manufacturer's instructions.
Combine flour, oats, sugar, baking powder, and salt in a large bowl; set aside. Beat egg yolks together in a bowl; add buttermilk, milk, and butter. Beat until well combined. Beat buttermilk mixture into flour mixture until well combined.
Beat egg whites and vanilla extract together in a bowl until soft peaks form; fold 1/2 into buttermilk batter until combined. Repeat with remaining 1/2 egg whites.
Ladle batter into the preheated waffle iron; cook until waffles are golden and crisp and iron stop steaming, about 5 minutes. Transfer to a paper towel-lined plate to prevent sogginess. Repeat with remaining batter.