Preheat oven to 350°F. Spray 6 muffin cups of a muffin pan with cooking spray.
Heat a nonstick skillet over medium. Cook sausage 5-6 minutes or until no longer pink, stirring to crumble as it cooks.
Dice onion and pepper. Add oil to skillet with sausage and heat for 1 minute. Add onion and pepper. Cook for 3-4 minutes, stirring occasionally. Remove from heat.
Whisk eggs and egg white in a bowl with oregano, pepper, salt and milk. Roughly chop spinach and stir into egg mixture.
Divide sausage between 6 prepared muffin cups. Pour an even amount of egg mixture over the sausage. Bake for 15-16 minutes or until the eggs have set. Cool for 2-3 minutes. Loosen the edges, then remove.