Bring a large pot of water to a boil and add salt. Add Brussels sprouts and cook between 30 to 60 seconds, depending on how large and woody the sprouts are. Drain well. If not stir-frying right away, shock them in ice water and set aside.
Combine the oyster sauce, fish sauce, soy sauce, and white pepper in a small bowl, stirring well to combine.
Heat the wok over very high heat. Add peanut oil, swirling to coat the sides. When oil begins to smoke lightly, add the garlic. Take off the heat and sizzle garlic, stirring often, until fragrant, about 15 seconds. Return to the heat, then add sprouts and chilies. Stir-fry for 30 seconds. Add the oyster sauce mixture and cook until liquid has almost completely evaporated, about 45 seconds. Add the stock and sugar. Continue to cook until the Brussels sprouts are tender and the sauce as thickened slightly, about 30 seconds. Serve the vegetables with the sauce.