Preheat oven to 425°F (220°C). Toss quartered baby potatoes with olive oil, salt, and pepper. Spread on a baking sheet and roast for 25–30 minutes, or until golden and crisp.
In a large bowl, combine shredded chicken, buffalo sauce, softened cream cheese, and ranch (or blue cheese) dressing. Stir until creamy and well mixed.
Transfer roasted potatoes into a greased 9x13-inch baking dish. Spread the buffalo chicken mixture over the top, then sprinkle with cheddar and mozzarella cheeses.
Bake at 375°F (190°C) for 10–15 minutes, or until the cheese is melted and bubbly.
Remove from the oven and top with chopped green onions, and if desired, bacon or blue cheese crumbles. Serve hot.