Preheat the oven to 450°. Rub the fish fillets with 2 tablespoons of the oil and season generously with salt and pepper. Set the fish in an ovenproof skillet and roast in the oven for about 9 minutes, until the fillets are white and firm. Keep warm.
Meanwhile, in a large skillet, heat the remaining 2 tablespoons of oil over moderately high heat until shimmering. Add the green beans and stir-fry until they begin to brown, about 3 minutes; season with salt and pepper. Add the water to the skillet, cover and steam until the beans are crisp-tender, about 5 minutes. Uncover and cook until the water has evaporated, about 3 minutes longer. Add the garlic and cook until fragrant, then add the fish sauce, oyster sauce, sugar and 1/2 teaspoon of pepper and stir to coat the beans.
Transfer the beans to a platter and set the cod fillets on top. Add 1/2 cup of water to the skillet along with any juices from the fish and boil, scraping up any bits from the bottom of the pan. When the sauce has thickened slightly, stir in the scallions and cilantro leaves. Pour the sauce over the fish and serve.