Preheat oven to 425°(F). Line a 10x15-inch rimmed baking sheet with foil; completely coat with 1 tablespoon of olive oil.
Put the potatoes in a large pot; add enough salted water to cover the potatoes by 2-inches. Bring to a boil over high heat; reduce heat to medium. Simmer, stirring occasionally, until fork tender. About 12–15 minutes. Drain well, and arrange in an even layer on the prepared baking sheet.
Using the bottom of a sturdy glass, flatten potatoes to ¼-to-½ inch thickness, taking care not to break them fully apart. Drizzle with remaining 2 Tbsp of olive oil. Use a pastry brush to be sure each potato is completely covered. In a small bowl, combine salt, pepper, and garlic powder. Sprinkle over potatoes.
Roast potatoes, turning after 25 minutes with a flat spatula, until crispy and brown. Bake an additional 15–20 minutes until golden brown on top. Let cool slightly.
Meanwhile, for the dressing, stir together sour cream, mayonnaise, and ranch seasoning in a large bowl. Stir in green onions, bacon, celery, and blue cheese.
Add the roasted potatoes to the bowl with the dressing. Gently toss until well coated. Drizzle with buffalo sauce, or serve on the side. Garnish with additional bacon and blue cheese. Serve warm or chill, covered, until ready to serve, up to 3 days.