Score the skin of the duck, place them skin-side down in a cold heavy-based frying pan. Turn the heat to high and let the duck slowly warm; this will render the fat from the skin, so you get a better texture and flavour, about 10 mins.
Meanwhile, bring a pot of water to the boil and add a dash of the oyster sauce to it. Boil the bok choi for 2 mins until wilted, drain and set aside. Reduce the heat under the duck, cook for a few more mins, then remove from the pan. When cool enough to handle slice the duck thinly.
Pour 1 tbsp of the fat from the duck into a wok with the oil and place over a high heat. Throw the duck in to the hot fat and let it cook for 3 mins until coloured. Add the bok choi and oyster sauce, stir well, simmer for 2 mins and serve.