Combine the garlic and ginger in a small bowl.
Preheat a wok over high heat for 10 minutes. Have 2 tablespoons of cold water in a small bowl next to the wok. Add the oil to the wok, and just as it starts to smoke, add the garlic and ginger. Stir-fry for a few seconds until they start to brown, and then immediately add the water.
Add the watercress and toss with tongs to coat with the oil. Cook just until it starts to wilt, about 1 minute. Add the oyster sauce and toss to coat. Remove from the heat and toss for another 30 seconds or so, until the watercress is completely wilted (the stems should be bright green and the leaves darker but not black). Transfer the watercress to a platter, and serve with rice.
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