Preheat the oven to 350F/176C. Spray a 10" bundt pan and dust with cocoa powder, then set aside.
In a large bowl, whisk together the flour, baking soda, cocoa powder, and salt. Set aside.
Using a handheld or stand mixer fitted with the paddle attachment, beat the butter on high speed until smooth and creamy. Add the sugar and beat on high speed until well combined. Add the oil, eggs, and vanilla and beat until combined.
With the mixer on low speed, add the dry ingredients in three additions alternating with the buttermilk, beginning and ending with the dry ingredients. Fold in the Oreo chunks.
Pour the batter into the prepared bundt pan and bake for 50 - 55 minutes or until a toothpick inserted into the centre comes out clean with moist crumbs. Let the cake cool for 20 minutes in the pan on a wire rack before removing. Leave to cool completely before frosting.
For the Glaze
Whisk together the icing sugar, milk, and vanilla until smooth. Add more milk if too thick. Pour over the bundt cake, and sprinkle with Oreo chunks.
TIP: If you can't get hold of buttermilk, add 1 tablespoon of white vinegar into a measuring jug, then pour in the milk. Leave for 5 minutes, then it's ready to use!