Heat the olive oil in a large pot over medium to medium-low heat.
Add the thinly sliced chicken, chopped onion, chopped celery, and chopped carrot to the pot; season with salt and pepper and cook until the chicken is no longer pink, about 5 minutes.
Stir in the garlic and cook for about 30 seconds until fragrant.
Pour in the chicken broth, add the uncooked orzo and minced parsley, then reduce the heat to low so the mixture simmers gently.
Simmer uncovered, stirring frequently to prevent sticking, until the orzo is al dente and the liquid is mostly absorbed, about 15 minutes.
Taste and adjust seasoning with salt and pepper, then serve warm.