Heat margarine and oil in a heavy aluminum saucepan. Add flour. Make a very dark roux. Constant stirring is the key to this, to prevent burning. Burned roux is unsalvageable! The pace of browning picks up the longer you cook.
Have the green onions and celery chopped and ready, and when the roux gets to the right color, remove the pan from the oven and stir them in. Keep stirring until the vegetables are soft. Add salt, peppers, oregano, thyme, parsley, lemon juice, and Worcestershire sauce.
Place four to six oysters in ovenproof baking dishes (au gratin dishes are perfect), along with two Tbs. of chopped artichoke. Top with enough sauce to generously cover. Bake in a 350 degree oven for about 15 minutes. Serve with hot French bread on the side.