Preheat the oven to 375°F (190°C).

In a mixing bowl, whisk together the eggs, salt, pepper, and oregano.

Add the chopped vegetables and cheese to the egg mixture and stir well.

Heat a skillet over medium heat and pour in the egg mixture.

Cook for about 5 minutes until the edges start to set.

Transfer the skillet to the preheated oven and bake for 20 minutes or until the frittata is set.

Let it cool for a few minutes before slicing and serving.