Spray a large non stick skillet generously with cooking spray. Heat skillet over medium high heat. Add potato, onion and zucchini and saute until golden brown, about 8 minutes. Stir in tomato, oregano, salt and pepper; keep warm. Whisk eggs with milk in a large bowl; season with salt and pepper. Spray a 10 inch (25 cm) non stick skillet with cooking spray. Heat skillet over medium high heat. Pour in egg mixture and reduce heat to medium low. As mixture begins to set, gently move spatula across bottom and sides of skillet to form large, soft curds. Cook until eggs are set but still moist. Divide scrambled eggs between 4 warm plates. Top each plate with an equal amount of ratatouille.