Preheat oven to 400 degrees F. Line a baking sheet with tin foil and cover with a thin layer of oil spray.
Combine and mix chopped cooked chicken, salsa, cumin, oregano, a ¾ cup of cheese, and ¾ of the minced green onion in a mixing bowl. Place about 1/3 cup of mixture in the center of each tortilla. Fold left and right sides inward, then roll from the bottom upwards so that the mixture is secure inside of the tortilla. Place seam-side down on the baking sheet.
Brush the tops and sides of the chimichangas with melted butter or margarine. Bake for about 25 minutes, until golden brown. Use remaining cheese and green onion as a garnish on top of the chimichangas. Serve with lettuce, tomato, guacamole, and/or sour cream if desired.