Cook spaghetti according to package directions; drain. In a skillet, heat oil over medium heat. Add onions, bell peppers, garlic, and chicken. Cook until chicken is no longer pink and vegetables are tender. Stir in crushed tomatoes, basil, oregano, salt, black pepper, and red pepper flakes. Cook 2 minutes longer. Add spaghetti and toss to coat well. Sprinkle with parmesan cheese.