In a blender or food processor, grind the oats until they have turned into fine flour, about 15-20 seconds.
Melt butter and let it cool to room temperature.
Preheat your waffle iron according to the manufacturer's instructions.
In a large bowl, whisk together oat flour, baking powder, salt and sugar (optional).
In a separate bowl whisk together egg and milk.
Gradually add the wet ingredients to the dry ingredients, whisking until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the melted butter and continue whisking until combined.
Lightly grease the waffle iron with cooking spray or butter. (This batter thickens quickly compared to traditional batter made with regular flour, so avoid letting it sit for too long.)
Pour the batter onto the waffle iron, using the recommended amount for your iron's size,and cook until golden brown and crispy.
Continue the process with the remaining batter, placing the cooked waffles on an oven rack to prevent sogginess as they cool slightly.