Preheat your oven to 350°F and lightly grease a standard 6-cup muffin tin with the olive oil.
Finely chop the baby spinach, quarter the cherry tomatoes, and mince the red onion.
In a large mixing bowl, whisk together the whole eggs and egg whites until well combined and slightly frothy.
Stir in the chopped spinach, tomatoes, red onion, dried oregano, sea salt, and black pepper.
Divide the egg mixture evenly among the 6 prepared muffin cups, filling each about three-quarters full.
Crumble the feta cheese over the top of each egg cup.
Bake for 20-25 minutes, or until the eggs are set and the tops are golden brown.
Allow the cups to cool in the pan for 5 minutes before using a knife to gently release them from the edges.