In a medium cast-iron skillet or another oven-proof pan, warm oil over medium heat. Add spinach and cook until wilted, around 1-2 minutes. Add the egg mixture. Sprinkle with feta.
Place skillet in the oven and bake until frittata is set; around 10 minutes.
Remove from oven and cut into quarters. Before serving, sprinkle each frittata slice with chopped tomato and oregano.
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