Heat lg. skillet over med. heat until hot. Add chicken, onion, salt, oregano and black pepper. Cook and stir about 6 min. or until chicken is no longer pink; set aside. Cover 15 1/2x10 1/2 in. baking pan with foil. Arrange muffins in single layer on prepared pan. Spread 2 tbs. pizza sauce on each muffin half. Cover generously with chicken mixture, dividing evenly. Cut 12 slivers bell pepper into "smiling" mouth shapes; set aside. Chop remaining bell pepper; sprinkle over mini-pizzas. Combine Cheddar and Mozzarella cheeses in sm. bowl; sprinkle generously over mini-pizzas. Bake at 450 degrees 12 min. or until cheese is light brown. Make face on each pizza by using 2 olive slices for "eyes" and 1 pepper shape for "mouth."