Set a large bowl of ice water by the sink and place a colander in the sink. Set a large pot of lightly salted water over high heat and bring to a boil. Add the Brussels sprouts and cook until they turn bright green but still retain plenty of crunch, about 1 minute. Drain, discarding the soaking liquid, then immediately transfer the brussels sprouts to the ice water to cool completely. Once cold, drain again, discarding the ice water, and set aside.
In a small bowl, add the oyster sauce, fish sauce, soy sauce and white pepper; stir well, and set aside.
Heat a wok over high heat, add the oil, and swirl it in the wok to coat the sides. When the oil begins to lightly smoke, add the garlic, then immediately remove the wok from the heat and let the garlic sizzle, stirring often, until it is fragrant but not colored, about 15-30 seconds. Return the wok to the the heat, add the brussels sprouts and chiles, and cook, stirring constantly and tossing the ingredients for 30 seconds to coat the sprouts with the garlic oil. Add the oyster sauce mixture and cook, stirring and tossing frequently, until the sprouts are tender but still crunchy and the liquid in the pan has almost completely evaporated, about 3 minutes. Add the pork stock then the sugar and continue stir-frying until the sauce has slightly thickened but is still very liquidy, about 1 minute more. Transfer the brussel sprouts to a large platter and serve hot or at room temperature.