Preheat oven to 350°F. Lightly grease a bundt pan and set aside.
To make the topping: Combine the sugar, flour, salt, cinnamon, and walnuts. Add the melted butter, stirring just until well combined. Set the topping aside.
In a medium size bowl, whisk together pumpkin puree, yogurt, eggs, and vanilla until combined. Add sugar and brown sugar.
In a large bowl, combine flour, oat flour, flaxseed meal, baking soda, baking powder, salt, and spices.
Add wet ingredients to dry and mix until just combined.
Pour into prepared bundt pan.
Sprinkle the topping over the batter.
Bake 30-35 minutes, or until toothpick inserted into the center comes out clean.