Preheat oven to 325 degrees F (165 degrees C).

Melt margarine in a large skillet over medium heat, and cook and stir the onion and celery until the celery is tender, about 10 minutes. Stir in the mushrooms, and cook and stir until they are tender and release their juice, 5 to 8 minutes.

In a bowl, mix the bread crumbs, egg, and oysters together, and stir in the cooked vegetables with juices and margarine, chicken broth, poultry seasoning, sage, salt, and black pepper until well combined. Place stuffing into a 2-quart casserole dish.

Bake in the preheated oven until the stuffing is hot, the oysters are cooked, and the top is browned, about 45 minutes.