Preheat oven to 350°. Line the bottom of an 8" or 9" loaf pan with parchment paper. Optional: Prepare topping by combining the flour, oats, sugar, and cinnamon in a small bowl. Then work the butter into the dry ingredients with your fingers until evenly mixed and a crumbly consistency in reached. Set aside until ready to use.
In a large mixing bowl, stir apple, sugar, coconut oil, eggs, buttermilk and vanilla until well blended. In a separate bowl, mix remaining dry ingredients until thoroughly blended.
Add dry ingredients to the wet mixture and stir until well incorporated. Pour into prepared loaf pan; top with sliced apples in a pattern around the edge of the loaf and sprinkle with streusel topping.
Bake for 50-60 minutes, or when a toothpick inserted in the center comes out clean. Cool for 5 minutes in the pan then run a sharp knife around the edge of the pan to release the sides. Gently turn over onto a wire rack and roll upright--this can be a little tricky--replace any lost streusel topping and cool completely on the wire rack before slicing.