For the crumble topping: Combine the oats, butter, brown sugar and cinnamon in a small bowl; set aside.
For the bread: Preheat the oven to 350 degrees F. Cut out an 8-by-4-inch rectangle of parchment paper and put in the bottom of a 9-by-5-inch loaf pan. Spray the pan and paper with cooking spray.
Put the oats in a blender and blend to a flour-like consistency. Combine the oat flour, wheat flour, cinnamon, baking powder, baking soda and salt in a large bowl.
Combine the honey, oil, vanilla and eggs in a medium bowl; add the sugar, stirring until combined. Add the apples and stir until well combined. Add the flour mixture and stir just until combined. Gently stir in the walnuts.
Pour the batter into the prepared loaf pan and sprinkle with the crumble topping. Bake until a wooden pick inserted in the center comes out clean, about 1 hour. Cool in the pan on a wire rack 10 minutes, then remove from the pan to the rack and cool completely.