Heat a little of the oil in a wok and toast the cashew nuts until they start to turn golden. Remove, then add the rest of the oil. Stir-fry the broccoli for 2 mins. Add water to the pan, cover with a lid and steam for 4 mins. Push to the side of the pan, then pour the oyster sauce into the other side. Bring to the boil and then stir into the broccoli. Toss in the cashews and serve.