Heat olive oil in a large pot over medium heat. Add chicken and cook until lightly golden and fully cooked. Stir occasionally so it cooks evenly.
Add onion, garlic, carrots, and celery. Cook until vegetables soften and become fragrant, about 4–5 minutes.
Pour in chicken broth and add salt, pepper, and oregano. Stir well and bring to a gentle boil.
Add orzo pasta and simmer, stirring often, until pasta is soft and tender.
Turn off heat and stir in lemon juice and lemon zest for fresh flavor.
Add parsley, taste, adjust seasoning if needed, and serve warm.