Rinse duck and pat dry; rub with salt, pepper, and poultry seasoning.
Melt butter in a small skillet over medium heat. Saute onion and celery in butter until tender. In a medium bowl, combine with apple and cornbread crumbs. Mix together to make dressing (if necessary, add a little water to moisten).
Preheat oven to 350 degrees F (175 degrees C).
Fill the duck's cavity with dressing, and sew shut with kitchen twine. Rub outside of bird lightly with olive oil, and place in a shallow roasting pan or 9x13 inch baking dish.
Bake in preheated oven for 60 to 80 minutes, or until internal temperature reaches 180 degrees F (80 degrees C.)