Heat the olive oil in a large pot over medium heat. Add the onion, carrots, and celery. Cook for 5 minutes until the vegetables soften.
Stir in the garlic and cook for 1 minute more. Season the chicken breasts with salt and pepper, then add them to the pot.
Pour in the chicken broth. Bring the mixture to a simmer and cook for 15 minutes until the chicken is cooked through.
Remove the chicken from the pot. Shred it with two forks and return it to the soup.
Add the orzo and simmer for 8 to 10 minutes until the pasta is tender. Stir in the lemon juice, lemon zest, and dill. Taste and adjust salt and pepper.