1Beat eggs, milk, salt and pepper with fork or wire whisk until a uniform yellow color. Stir in cheese; set aside.
2Heat oil in 10-inch ovenproof skillet over medium heat. Cook potatoes and oregano in oil 3 minutes, stirring frequently. Stir in spinach. Cook about 1 minute, stirring frequently, until potatoes are tender and spinach starts to wilt. Reduce heat to medium-low.
3Pour eggs over potato mixture. Cover and cook 7 to 10 minutes or until eggs are set almost to center and are light brown on bottom.
4Set oven control to broil. Broil frittata with top about 5 inches from heat about 2 minutes or until top just starts to brown.