Wash and dry the chickens and season them with salt and pepper. Rub the skin all over with butter. Mince the onion and cook it briefly in butter. Add to the breadcrumbs and season with the other ingredients (except oysters). Cut the oysters in two and then add them and their liquor to the mixture. Spread the stuffing on the inner side of each piece of chicken. Lay the pieces on a rack in the center of the oven, preheated to 400 degrees F. Put a large pan in the bottom of the oven to catch the drippings and keep the stove clean. Cook the chicken for 10 minutes, then reduce the heat to 350 degrees F and cook for 20 minutes longer. Brush the surface with a little butter and serve.