Preheat oven to 350°F with a rack in the center of the oven. Set aside a 12-cup bundt pan (don't grease it just yet).
In a large bowl whisk flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter, oil, sugar, and orange zest on medium speed for 2-3 minutes, until light and fluffy. Add eggs one at a time, followed by vanilla extract. Beat until combined, scraping the sides and bottom of the bowl as needed.
In a liquid measuring cup combine orange juice and buttermilk.
Add one third of the dry ingredients to the sugar/egg mixture, beat until just combined. Add half of the buttermilk, beating until just combining. Repeat adding half the remaining flour, all the remaining buttermilk, then the last bit of flour, beating between each addition. Stir with a rubber spatula to make sure it is completely combined.
Thoroughly grease a 12-cup bundt pan (I use nonstick spray with flour in it). Pour the batter into the pan and smooth into an even layer. Bake for 40-50 minutes, until a toothpick inserted into the center comes out clean. Place the pan on a wire rack to cool for 30 minutes (don't invert it yet). Proceed with syrup as you approach the end of the 30 minute cool time.