Preheat oven to 300°. Divide bread cubes between 2 baking sheets and toast in oven, stirring occasionally, until dry and crisp, about 30 minutes. Let cool on sheets, then pour into a large bowl. Increase oven temperature to 350°.
Meanwhile, cook Chinese sausage in a large heavy frying pan over medium heat until edges begin to curl and fat renders, about 4 minutes. With a slotted spoon, transfer sausage to bowl of bread cubes, leaving fat behind (if there isn't any fat, add 2 tbsp. oil). Increase heat to medium-high, add shiitakes, and cook until golden, about 5 minutes. Season lightly with salt and pepper, then scoop into bowl of sausage and bread cubes.
Wipe out pan. Add bacon and cook over medium heat until browned and fat has rendered, about 8 minutes. Using a slotted spoon, transfer bacon to bread cube mixture, leaving fat in the pan. Discard all but 2 tbsp. fat.
Add leeks, celery, and carrots to pan and cook over medium heat until softened, about 10 minutes. Add chestnuts and chopped herbs, then season with salt and pepper. Add butter to pan and allow to melt completely.
Stir vegetable mixture into bread cube mixture. With kitchen scissors, snip oysters into 1/2-in. to 1-in. pieces and add to bowl. Add stock, season with salt and pepper, and mix thoroughly.
Butter a 3 1/2- to 4-qt. baking dish. Transfer stuffing to dish and cover with foil. Bake 30 minutes, then remove foil and bake until golden and crispy on top, 30 to 40 minutes more.
Serves 10 to 12