Heat oil over medium heat. Add the onion to the pan and cook for 5 or so minutes, until soft and translucent.Add the mushrooms to the pan, and cook for 3 more minutes, until they are soft and cooked through. Add the spinach and cook for 2 more minutes, until soft and the spinach is wilted.While the veggies are cooking, beat the eggs with the salt, pepper, and oregano in a bowl.Add the eggs to the pan and cook for 3-4 more minutes, until the eggs are cooked through.Fold in the feta cheese and remove from heat and cool completely.Assemble the burritos: Spread out ¾ cup of the *cooled* egg/veggie filling onto a tortilla. Sprinkle with ¼ cup of shredded cheese. Roll up, sealing both ends, and wrap firmly in plastic wrap.

Notes: If you want to make the tortilla crispier, fry in a frying pan for 3-4 minutes each side.