Preheat the oven to 350°F. Grease a 9x13 inch baking dish with olive oil.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, and oregano until well combined.
Add the diced bell peppers, tomatoes, spinach, feta cheese, red onion, and black olives to the egg mixture. Stir until all ingredients are evenly distributed.
Pour the mixture into the prepared baking dish, spreading it out evenly.
Bake in the preheated oven for 30-35 minutes, or until the eggs are set and the top is lightly golden.
Remove from the oven and let it cool for 5 minutes before slicing. Serve warm.