Preheat the oven to 425°F. Place the potatoes on a rimmed baking sheet and drizzle over the olive oil. Toss to coat then spread the potatoes out on the baking sheet and bake for 20 to 25 minutes, until golden brown and tender. Cool slightly.
While the potatoes are cooking, in a small bowl combine the mayonnaise, sour cream, harissa, and salt and pepper.