In a large pot, heat olive oil and add chopped chicken, onions, and garlic. Add Italian seasoning and cook until no longer pink. Pour in the chicken broth, carrots, celery, and tomatoes. Add the oregano, parsley, thyme, and basil. Meanwhile, cook tortellini separately according to package directions. Simmer, stirring occasionally, until vegetables are soft, about 15 minutes. Add cooked tortellini, Parmesan cheese, and freshly ground black pepper if desired.