FOR THE OAT STUFFING: Cook and stir onion in margarine in 10-inch skillet over medium heat until light brown. Stir in remaining ingredients. Cook and stir until oats are golden brown and crisp, 3 to 4 minutes.
PREPARE OAT STUFFING; Fill wishbone area of chicken with stuffing. Fasten neck skin to back with skewer. Fold wings across back with tips touching. Fill body cavity lightly. (Do not pack; stuffing will expand during cooking.) Tie or skewer drumsticks to tail.
Place chicken, breast side up, in shallow roasting pan. Arrange onions around chicken. Brush chicken and onions with margarine. Roast uncovered in 375 oven, brushing chicken and onions several times with remaining margarine, until chicken and onions are done, about 1 1/2 hours. 6 servings.