In a large pot, heat the olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for about 5-7 minutes, or until the vegetables are softened.
Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Pour in the chicken broth and bring to a boil.
Add the uncooked orzo pasta and cook according to package instructions, usually about 8-10 minutes, until al dente.
Stir in the shredded chicken, lemon juice, and dried oregano. Season with salt and pepper to taste. Heat through for about 2-3 minutes.
Serve hot, garnished with fresh parsley and lemon slices.