Preheat oven to 450 degrees F. In a baking dish, toss cauliflower florets with olive oil and season with salt and pepper.
Bake until tender for 20 minutes. Reduce oven temperature to 350 degrees F.
Combine cream cheese, buffalo sauce, ranch, green onions, and garlic powder in a medium bowl. Set aside.
Toss rotisserie chicken with roasted cauliflower in baking dish and spread cream cheese mixture on top.
Top with cheddar and bake until cheese is completely melted and bubbly, 20 minutes.
Let cool 10 minutes, then drizzle with ranch and garnish with green onions or chives and serve.