Preheat waffle iron and lightly grease with cooking spray or oil.
In a large bowl, whisk together oat flour, protein powder, baking powder, cinnamon, and salt.
In another bowl, whisk eggs, milk, Greek yogurt, honey, and vanilla until smooth.
Pour wet ingredients into dry ingredients and stir until just combined (do not overmix). Batter should be thick but pourable—add a splash of milk if needed.
Pour batter into waffle iron and cook according to manufacturer’s instructions until golden and crisp (about 3–5 minutes per waffle).
Serve warm with fresh fruit, nut butter, or sugar-free syrup.