Preheat oven to 425F (215C) and line a 12-count muffin tin with paper liners.
Combine melted butter and canola oil in a large bowl. Add sugars and stir well.
Stir in eggs, vanilla extract, and buttermilk.
In a separate bowl, combine flour, baking powder, cornstarch, cinnamon, salt, and nutmeg. Add oats and stir again until combined.
Gently stir ingredients together using a spatula until just combined.
Divide batter into prepared pan, filling liners ¾ of the way full.
Prepare your streusel topping
Prepare streusel by whisking together flour, oats, sugars and cinnamon.
Pour the melted butter into the mixture and, using a fork, stir until crumbles form.
Using your hands, evenly divide streusel over muffin batter, piling the crumbly streusel over the batter and gently pressing the topping into the batter.
Transfer to 425F (215C) oven and bake for 8 minutes, then, without opening the oven door, reduce the oven temperature to 350 (175C) and bake another 12 minutes, or until tops are just beginning to turn golden brown.
Allow to cool and enjoy!