In a 5- to 6-quart Dutch oven cook carrots, celery, and onion in hot oil over medium heat for 5 minutes, stirring occasionally.
Stir in broth, cannellini beans, chicken, green beans, and 1/4 teaspoon pepper. Bring to boiling. Stir in pasta. Return to boiling; reduce heat. Simmer, uncovered, for 5 minutes.
Stir in zucchini and/or yellow squash. Return to boiling; reduce heat. Simmer, uncovered, for 8 to 10 minutes more or until pasta is tender and green beans are crisp-tender. Stir in tomatoes; heat through. Sprinkle each serving with oregano and additional pepper.