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 quarter cup meat filling. Spread thin layer of spaghetti sauce on bottom of 13 x 23 cm 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.