Put the Potatoes in a large pot with water to cover by at least 2 inches. Cover and bring the water to a boil. Continue boiling until the potatoes are tender, about 15 minutes. Drain, reserving ¾ cup of the cooking liquid. Return the potatoes to the pot. Coarsely mash the potatoes with a fork, adding enough of the reserved cooking liquid to moisten. Stir in the Parmesan cheese and the oil. Season to taste with salt and pepper and serve.