Preheat a waffle maker according to manufacturer’s directions.
Whisk whole wheat flour, all-purpose flour, oat flour, sugar, baking powder, and salt together in a bowl. Whisk egg yolks, milk, canola oil, and vanilla extract in a separate bowl. Beat egg whites in a third bowl (not plastic) with an electric mixer until soft peaks form when you lift the beaters straight out of the beaten eggs, 1 to 2 minutes.
Make a well in the dry ingredients and pour the milk mixture into the well; mix batter until smooth. Gently fold the egg whites into the batter, retaining as much volume as possible. Let batter stand until slightly thickened, about 5 minutes.
Pour 1/2 cup of batter, or amount recommended by the manufacturer, onto the waffle maker; close lid and bake waffle until browned and crisp, 2 to 4 minutes.
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