Turn on your instant pot to the sauté option, add oil and butter and wait for the butter to melt.
Add chicken breasts and cook briefly for about 2 minutes in each side.
Stir in onions, celery and carrots and keep cooking for about 3 minutes more.
Stir in garlic and cook for another 30 seconds then add oregano, basil, thyme and black pepper and mix well
Add in about a cup of chicken broth scrapping the bottom of the pot to get bits unstuck.
Pour in the remaining stock and rice mixing all well.
Turn the sauté option off and cover with lid.
Move the valve to seal option and pressure cook on high for 12 minutes.
Manual release the pressure in bursts then carefully remove the lid.
Take the chicken out and shred with fork.
Return the chicken to the pot keeping it in the keep warm setting.
Taste and adjust to see if you need more salt.