Heat the oven to 350° F. Oil, butter, or spray a 9-inch cake pan that is at least 2 inches deep with cooking spray and line the bottom with parchment paper. (If your cake pan is less than 2 inches deep, divide between 2 pans and start checking for doneness at 30 minutes.)
In a bowl, whisk the flour, sugar, salt, baking soda and powder. In another bowl, whisk the olive oil, milk, eggs, orange zest and juice and Grand Marnier. Add the dry ingredients; whisk until just combined.
Pour the batter into the prepared pan and bake for 1 hour, until the top is golden and a cake tester comes out clean. Transfer the cake to a rack and let cool for 30 minutes.
Run a knife around the edge of the pan, invert the cake onto the rack and let cool completely, 2 hours.