Heat olive oil in a large pot over medium heat. Add onions, carrots, and celery. Cook until onions are translucent, about 5 minutes.
Pour in chicken broth and bring to a boil. Add chicken breasts, rice, salt, and pepper. Reduce heat to a simmer.
Cover and let simmer for 20-25 minutes, until the rice is tender and the chicken is cooked through.
Remove chicken from the pot, shred it, and return it to the soup. Adjust seasoning with salt and pepper.