Heat oil in a large skillet over medium heat. Add onion and celery; cook, stirring, until soft, about 10 minutes. Add sausage, poultry seasoning, pepper and salt; cook, stirring, until no pink is visible, 6 to 8 minutes more.
Transfer to a 5- to 6-quart slow cooker. Add bread, apple and broth; stir to coat well. Cover and cook on High for 2 hours.
Tip: If using fresh bread: Preheat oven to 275°F. Spread bread on a large rimmed baking sheet and bake until dry to the touch, about 30 minutes. Let cool before using.