Cut each piece of chicken in half horizontally. Sprinkle the chicken with oregano. Season. Heat half the oil in a large frying pan over high heat. Cook the chicken for 2 mins each side or until golden brown. Transfer to a large heatproof plate.
Heat remaining oil in the pan. Add the onion and garlic. Cook for 1 min or until onion softens. Add cherry tomato and diced tomato and bring to the boil.
Return the chicken to the pan with the olives and bocconcini. Reduce heat to medium-high and cook for 8 mins or until the chicken is cooked through and sauce thickens slightly. Serve drizzled with pesto.