Preheat oven to 400F. Grease a baking sheet with cooking spray.
Cut potatoes into uniform, 1/4-inch fries using a knife or a French fry cutter (I like leaving the peels on, but peel if desired). In a large bowl, toss with oil (save the bowl for later, no need to wash it yet). Spread fries onto greased baking sheet and bake until tender and golden brown, about 20 minutes.
Return fries to the bowl and toss with buffalo sauce. Spread back onto the baking sheet and evenly top with cheese. Bake until cheese is melted and bubbly, about 4-5 minutes. Sprinkle with chives and serve immediately with Ranch dressing alongside for dipping.