I've tried a variety of different flours in baking brownies and I like all purpose flour and oatmeal flour the best. Almond flour is also great to use but gives the brownies a slightly grainer texture. I've also made avocado flourless brownies and sweet potato flourless brownies that taste great too! It's all about personal preference. If you've wondered if you can substitute all purpose flour for oatmeal flour in this recipe, I would not recommend it. Oat flour does not have gluten and also sucks up moisture much more than all purpose flour.