In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 5 minutes until translucent.
Stir in the minced garlic, diced carrots, and celery. Cook for an additional 5 minutes, stirring occasionally.
Pour in the chicken broth and bring the mixture to a boil.
Once boiling, add the orzo pasta and cook according to package instructions, usually about 8-10 minutes.
Reduce the heat to low and stir in the shredded chicken, lemon juice, and dried oregano. Season with salt and pepper to taste.
Allow the soup to simmer for 5 minutes to heat through.
Serve hot, garnished with fresh parsley and lemon slices.