Spray a large non-stick skillet generously with cooking spray. Heat skillet over medium-high heat. Add potato, onion and zucchini and sauté until golden brown, about 8 minutes. Stir in diced 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.