Pour the wet ingredients into the dry ingredients. Stir with a big spoon until just combined (the batter will still be a little lumpy). Let the batter rest for 10 minutes so the oat flour has time to soak up some of the moisture. Plug in your waffle iron to preheat now.
In a mixing bowl, whisk together the dry ingredients: oat flour, baking powder, salt and cinnamon. In another bowl, whisk together the wet ingredients: milk, melted coconut oil or butter, eggs, maple syrup and vanilla extract. (If your coconut oil solidifies on contact with cold ingredients, gently heat the wet mixture in the microwave in ten seconds intervals, until it melts again.)