Pour oil into a heavy 10 to 12-inch frying pan on medium-high heat; when oil is hot (about 325°), add flour all at once. Stir often until roux is a rich, deep caramel color, 7 to 8 minutes. Off the heat, dump in the onions and bell peppers all at once. Stir until foaming stops and vegetables are soft, about 1 minute. Meanwhile, in a 12 to 16-quart pan, bring broth to boiling; stir in the hot roux. If made ahead, cover and chill up until next day. Cover and reheat to simmering. De-vein shrimp in shell by sliding a thin wooden skewer into back and under vein, then gently pulling vein out; repeat in several places along back. Rinse shrimp; add all at once to broth with crab in shell, crab meat, sausage, and oysters (with liquid). Cover and simmer very gently so flavors mingle, 15 to 30 minutes. Add 2 tablespoons hot pepper seasoning. Transfer gumbo to a large serving bowl and sprinkle with parsley. Ladle portions over rice in wide soup bowls. Add liquid hot pepper seasoning and salt to taste. Makes 12 to 14 servings. Per serving: 670 cal.; 37 g protein; 28 g fat; 64 g carbo.; 719 mg sodium; 169 mg chol.