Prepare the fries: If using fresh potatoes, peel and cut into ¼-inch thick fries. Soak in cold water for 30 minutes, then pat dry. Toss with olive oil, salt, and pepper. If using frozen fries, spread evenly on a baking sheet.
Cook the fries: Preheat oven to 425°F (220°C). Bake fries for 20-25 minutes, flipping halfway, until golden and crispy.
Cook the bacon: In a skillet over medium heat, fry bacon for 8-10 minutes until crispy. Drain on paper towels and chop into small pieces.
Assemble the fries: Spread cooked fries on an oven-safe platter or baking sheet. Sprinkle shredded cheddar and mozzarella evenly over fries. Top with chopped bacon.
Melt the cheese: Bake at 425°F (220°C) for 5-7 minutes until cheese is melted and bubbly.
Garnish and serve: Sprinkle sliced green onions over fries. Serve immediately with sour cream or ranch on the side.