Heat oil in a Dutch oven or other large pot over medium-high heat. Add carrots, celery, onion and garlic; cook, stirring often, until the vegetables are just soft, about 8 minutes. Stir in broth, chicken, salt, Italian seasoning and pepper; bring to a boil over high heat. Reduce heat to medium and gently boil, undisturbed, for 5 minutes. Stir in orzo and cook, undisturbed, until the orzo is tender, 8 to 10 minutes.