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Combine the oat bran, whole wheat flour, all-purpose flour baking powder and salt.
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Combine the milk, oil, and egg yolk; add to the dry ingredients, stirring just until moistened.
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Beat egg whites at high speed with an electric mixer until stiff peaks form; gently fold into the batter.
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Coat a waffle iron with cooking spray, and allow waffle iron to preheat. Spoon 1 1/3 cups batter onto hot waffle iron, spreading batter to edges. Bake until lightly browned. Repeat with remaining batter.
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Serve immediately with desired toppings.