In a medium skillet, saute the onion and celery in butter until tender and fragrant, then transfer to a large mixing bowl.
Add the parsley, poultry seasoning, sage, and pepper. Mix with the onion and celery, then add the bread cubes.
In a separate bowl, combine the chicken broth, egg, and oysters. Pour over the bread mixture and stir gently to combine.
Transfer the mixture to a greased 1-quart baking dish. Cover with aluminum foil and bake at 350°F. After 20 minutes, removed the cover and bake for an additional 10 to 15 minutes, until stuffing is lightly browned or a thermometer reads 160°F.