Preheat your waffle iron. Lightly grease it if needed.
Blend the batter: Add oats, protein powder, egg, milk, butter or oil, sweetener, baking powder, vanilla, and salt to a blender. Blend on high for 20–30 seconds until smooth.
Scrape the sides and pulse once more if needed.
Rest the batter for 2–3 minutes to let the oats hydrate and the baking powder activate. The batter should thicken slightly.
Pour and cook: Pour batter into the center of the hot waffle iron, just enough to cover the surface. Close and cook until golden and crisp, usually 3–5 minutes depending on your machine.
Repeat with remaining batter.
Keep cooked waffles on a wire rack so they stay crisp.
Serve with yogurt, fresh berries, nut butter, or a drizzle of maple syrup.