Separate the bok choy into stalks. Wash in several changes of cold water and drain thoroughly in a colander. Trim 1/4 inch from the bottom of each stalk. Halve each stalk lengthwise and cut stalks and leaves into 2 inch long pieces. In a bowl, combine the stock, oyster sauce, soy sauce, cornstarch, and sugar. Heat a 14 inch flat-bottomed wok or skillet over high heat until hot but not smoking. Add 1 tsp. vegetable oil and 1 clove garlic, and stir-fry 10 seconds, or until garlic is fragrant. Add the remaining 1 tsp. vegetable oil, bok choy, and garlic, and stir-fry 1-2 minutes, or until leaves are just limp and bok choy is bright green. Restir the stock mixture and swirl into wok. Stir-fry 1-2 minutes, or until the sauce has thickened slightly and lightly coats the vegetables. Serve immediately.