Preheat your oven to 325 degrees F.
Evenly arrange bread cubes on 2 large rimmed baking sheets. Bake in upper and lower thirds of your oven for 30 minutes (or until lightly browned and crispy), switching pans after 15 minutes. Place baking sheets on a wire rack to cool.
While bread cubes are baking, cook bacon (in 2 batches) in a large skillet over medium to medium high heat until browned and crisp, turning as needed. *Reserve bacon fat between batches. Transfer cooked bacon to a plate lined with paper towels. Pour off bacon fat EXCEPT ¼ cup. Once the bacon has cooled, cut into about ½-inch pieces.
Add onions, celery, thyme, sage, garlic, salt, and pepper to the skillet with the bacon fat. Cook over medium high heat until soft, stirring frequently (about 7 minutes).
Place cooled bread cubes in a very large mixing bowl. Add onion and celery mixture, bacon pieces, parsley, melted butter, and chopped oysters. Gently stir or toss to combine.
Drizzle with ½ of the chicken broth and toss or gently stir to combine. Drizzle with the remaining ½ of chicken broth and toss again to combine.
Spray a 13 x 9-inch baking dish with cooking spray. Transfer the mixture to the baking dish. Cover with aluminum foil and bake on the center oven rack for 30 minutes at 325 degrees F.
Uncover and bake for an additional 30 minutes.