Cook manicotti according to package directions; drain. Cool in single layer on foil. Preheat oven to 350. In a skillet, heat oil over medium heat. Add chicken and onions. Cook until chicken is no longer pink and onions are tender. Stir in 1 cup mozzarella cheese, egg whites, bread crumbs, parmesan cheese, oregano, salt, and black pepper. Fill each cooled pasta tube with about 1/4 cup meat filling. Spread thin layer of spaghetti sauce on bottom of 13 x 9" pan. Place filled pasta in prepared pan; cover with remaining sauce. Sprinkle with remaining mozzarella cheese. Bake, covered, for 45 minutes or until hot and bubbly.