Lightly coat a medium skillet with nonstick cooking spray; add potato to skillet. Cook potato, covered, over medium heat about 5 minutes or until lightly browned and just tender, turning occasionally. Add sausage, sweet pepper, and green onion. Cook for 2 to 3 minutes, covered, or until sausage is heated through and vegetables are tender. Remove from heat. Arrange potato and other ingredients so they are evenly distributed over bottom of skillet.
Combine eggs; egg whites; milk; oregano; salt; and, if desired, black pepper. Pour over vegetable mixture in skillet; do not stir. Place skillet over medium heat. As mixture sets, run a spatula around edge of skillet, lifting edge with the spatula to allow liquid to run under frittata. Continue cooking and lifting edges until egg mixture is set (top will be wet). Remove from heat; let stand, covered, for 5 minutes. Loosen edges; transfer to a serving plate. To serve, cut into wedges. Makes 2 servings.