Put the potatoes in a large pot of water. Cover and bring water to a boil. Cook until potatoes are tender, about 15 min.
Drain, reserving 3/4 c. of the cooking liquid. Return the potatoes to the pot.
Coarsely mash the potatoes, adding enough reserved cooking liquid to moisten.
Using a large fork, stir in the Parmesan and the oil. Season the potatoes to taste and serve.