1. In a small bowl, soak bread in milk 5 minutes. Squeeze bread to remove excess liquid.
2. Crumble sausage meat into a small frying pan. Cook, stirring, over medium heat with 2 tablespoons water 2 minutes, or until no longer pink. Add onion, oregano, and bread; blend well. Remove from heat and set aside.
3. Preheat oven to 425°. Rinse chicken and pat dry inside and out. Add mozzarella cheese to sausage mixture. Spoon stuffing into cavity of chicken. Cross bone ends of legs and tie together with string. Rub skin with olive oil.
4. Place chicken in a roasting pan just big enough to fit. Roast 15 minutes. Turn heat down to 375° and roast, basting several times with any pan juices, 1 1/4 to 1 1/2 hours longer, or until juices run clear. Allow chicken to rest 10 minutes before carving.