Set Instant Pot to sauté mode and heat the oil.
Add onion, carrots and celery and sauté for several minutes until the onion is translucent. Add in minced garlic, stir and press cancel.
Pour in the chicken broth and scrape the bottom of the pot to deglaze the pan and dislodge anything that is stuck on the bottom.
Season the chicken breasts with salt and pepper and add to the liquid, making sure they are completely covered.
Cover and seal the Instant Pot and cook at high pressure for 6 minutes followed by an 8-minute natural pressure release.
Release any remaining pressure and open the Instant Pot. Remove the chicken breasts to a plate and cut into chunks or shred chicken with two forks. Set aside.
Return the instant pot to sauté mode, add the orzo to the liquid, and cook for about 6 minutes or until the pasta is cooked through.
Press cancel and return the chicken to the soup followed by the lemon juice and herbs, if using. Adjust seasonings and then serve.