Heat a large flat-bottomed wok (or large skillet) over high heat until a bead of water vaporizes within 1-2 seconds of contact. Swirl in the oil, add the garlic, and stir-fry 10 seconds or until the garlic is fragrant. Add the yu choy and stir-fry 30 seconds or until the pieces are lightly coated in the oil. Sprinkle on the pepper and stir-fry 1 minute or until the leaves are just limp and the yu choy is bright green. Add the oyster sauce mixture to the wok and stir-fry 30 seconds to 1 minute, or until the yu choy is just crisp-tender.