Preheat the oven to 350 (F). Spray the bottom of a 9-inch cake pan generously with cooking spray, before lining with a parchment paper round. Spray the parchment paper, and spray the sides of the pan generously. Set aside.
In a medium bowl, combine 1 3/4 cups granulated sugar and the freshly grated zest from 1 medium-large orange. Use your fingers to rub the zest into the sugar — doing so will release oils from the orange zest and infuse your sugar to become more flavorful and aromatic. Add 2 cups all-purpose flour, 1 1/2 teaspoons kosher salt, 1/2 teaspoon baking powder, and 1/2 teaspoon baking soda, whisking until all the ingredients are fully incorporated.
In a large bowl, combine 1 1/3 cups extra virgin olive oil, 1 1/4 cups whole milk, 3 large eggs, 1/4 cup fresh orange juice and 1/4 cup Grand Marnier. Whisk together until well combined, and the mixture is a uniform color.
Sprinkle the dry ingredients (from the 2nd step) over the surface of the wet ingredients (from the 3rd step). Use a rubber spatula to fold the dry ingredients into the wet ingredients, until the batter is smooth and one uniform color. Pour the batter into the prepared pan and bake for 1 hour, until the top of the cake is golden and a skewer inserted into the center of the cake comes out clean. Transfer the cake to a wire rack and allow to cool for 20 minutes, until running a knife around the edge of the pan and inverting the cake onto the wire rack to cool completely.