Preheat the oven to 425 degrees.
Combine the potatoes, olive oil, and salt in a 9 X 11-inch roasting pan and toss thoroughly. Roast the potatoes until fork-tender, about 35 to 40 minutes.
Transfer to a bowl and, using a wooden spoon, smash the potatoes up against the side of the bowl. Add the buttermilk and continue to smash the potatoes until the liquid is thoroughly combined. Season with the pepper and serve.