Preheat oven to 350°F (175°C) and generously grease a 12-cup muffin tin with non-stick cooking spray.
In a large mixing bowl, whisk together the eggs, milk, salt, pepper, and dried oregano until well combined.
Stir in the chopped spinach, crumbled feta, and sliced green onions.
Divide the egg mixture evenly among the 12 muffin cups, filling each about three-quarters full.
Bake for 20 to 25 minutes, or until the eggs are set and the tops are slightly golden.
Allow the muffins to cool in the pan for 5 minutes before using a thin knife or spatula to gently remove them.