In a large pot or Dutch oven, heat the olive oil over medium heat. Add the chopped onion, carrots, and celery. Cook until softened, about 5-7 minutes.
Add the chicken breasts to the pot and pour in the chicken broth. Bring to a boil, then reduce heat and simmer for 15-20 minutes, or until the chicken is cooked through.
Remove the chicken from the pot and shred it using two forks. Return the shredded chicken to the pot.
Stir in the orzo pasta, lemon juice, parsley, salt, and pepper. Cook for another 8-10 minutes, or until the orzo is tender.
Taste and adjust seasoning if necessary. Serve hot.
Notes
Garnish with extra parsley and a lemon wedge, if desired.