Position rack in upper third of oven; preheat broiler.
Whisk eggs, oregano, salt and pepper in a medium bowl. Heat oil in a large, ovenproof, nonstick skillet over medium heat. Add bell pepper and scallions and cook, stirring constantly, until the scallions are just wilted, 30 seconds to 1 minute.
Pour the egg mixture over the vegetables and cook, lifting the edges of the frittata so uncooked egg can flow underneath, until the bottom is light golden, 2 to 3 minutes. Dot the top with cheese, transfer the pan to the oven and broil until puffy and lightly golden on top, 2 to 3 minutes. Let rest for about 3 minutes before serving. Serve hot or cold.
Make Ahead Tip: Let cool, cover and refrigerate for up to 1 day; serve cold.
1 vegetable, 1 1/2 medium fat meat, 1 1/2 fat