The olive topping on our cod was inspired by tapenade; it includes olives, anchovies, capers, olive oil, and lemon juice. Use a ceramic or glass baking dish; an aluminum dish could react with the acidic lemon juice and discolor the fish.
Preheat oven to 400 degrees. Butter a 9-by-13-inch nonreactive baking dish; set aside. Stir together olives, capers, anchovies, garlic, lemon juice, oil, and oregano in a small bowl; set aside.
Season fish with salt and pepper; place in baking dish. Scatter olive mixture evenly over fish. Top with lemon slices.
Cover dish with foil. Bake until fish is firm, flaky, and cooked through, 15 to 17 minutes. Let rest 5 minutes before serving. Serve drizzled with juices from dish. Garnish with oregano leaves.