My first thought would be that you made smaller cookies than I did and didn’t shorten the baking time, if you’re oven runs hot that will bake the cookies quicker, if you baked the cookies until they looked fully baked, that would result in a dry cookie as they continue to cook as they cool. This oatmeal cookie recipe is adapted from the Quaker Oats recipe, but I made mine cookies much larger, so the baking time is longer. Other reasons for dry cookies, thin baking sheets heat quickly and could over bake cookies, dark (nonstick) baking sheets dry out cookies, too much flour. Do any of those seem like the reason?