To have a crispy crumb on top of the baked apple, take care of a few things while making the topping. First, evenly distribute the butter with the rolled oats, flour, and sugar. And do not go light with the butter. Butter helps bake and crunch everything, so we need little bits of butter all over the crumbs.   Second, you must bake for the recommended time and at the right temperature. If baking is less, the topping mixture hasn't had enough time to bake and crisp.   As recommended in the recipe, the mixture of flour, oats, sugar, and butter is important.