extra-virgin olve oil • sweet Italian sausage, casings removed, broken into 3/4 inch pieces • bone-in chicken breast halves, skin removed, halved • coarse salt and ground pepper • yellow onion, diced • waxy potatoes, halved or quartered if large • stalks celery, cut into 1 1/2 inch pieces plus leaves for garnish • red wine vinegar • fresh oregano leaves