Preheat oven to 350 degrees F (175 degrees C). Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Separate shells and let stand, so they are not touching, on waxed paper or foil.
In large skillet over medium heat, combine chicken, onion, egg and bread crumbs. Cook until chicken is no longer pink. Season with oregano, basil, salt and pepper. Stir in three-quarters of the shredded mozzarella and cheddar; cook until cheeses melt.
Fill shells with chicken mixture, using a tablespoon, and place in a 9 x 13 baking dish. Cover the shells with the tomato sauce and top with the reserved cheese. Cover with foil and bake 45 minutes, or until lightly browned and bubbly.