Add the olive oil to the base of the instant pot. Add the chicken thighs skin side down and set to sauté until well browned – about 4 minutes. Turn the thighs and brown the other side. Remove from the instant pot with a slotted spoon and set to one side.
Whilst the chicken is browning you can prepare the vegetables. Thinly slice the garlic and onion and slice the bell peppers into wide strips. Once you have browned the chicken, add the peppers, onion and garlic to the instant pot along with the lemon juice and oregano. Sauté for 3-4 minutes until tender and fragrant.
Return the chicken thighs to the pot and arrange on top of the vegetables. Add the stock. Season with salt and pepper if desired. Place the lid on the instant pot and cook at high pressure for 8 minutes. When the thighs have finished cooking, do a quick release as per the instant pot guidelines. Serve hot.