How to Make It
Heat oven to 350° F. Oil an 8-by-4-inch loaf pan and dust with flour, tapping out the excess.
In a large bowl, whisk together the flour, cornmeal, baking powder, baking soda, and salt. In a medium bowl, whisk together the olive oil, buttermilk, eggs, and 1½ cups of the sugar; add to the dry ingredients and mix just until combined (do not overmix).
Transfer the batter to the prepared pan and bake until golden and a toothpick inserted in the center of the cake comes out clean, 50 to 55 minutes. Let cool for 20 minutes in the pan, then transfer to a wire rack to cool completely.
Meanwhile, cut away the peel and pith of the oranges: Working over a small bowl, cut along both sides of each orange segment, releasing the segments into the bowl. Toss the orange segments with the vanilla and the remaining ¼ cup of sugar. Let sit for at least 30 minutes and up to 2 hours, tossing occasionally.
Serve the cake, sliced, topped with the oranges and mascarpone.