1.Wash the collard greens and trim the hard ends off. Cut the leaves into small pieces, approximately 2 inches by 2 inches.
2.Prepare a large bowl of ice water and set aside.
3.Bring a pot of water to boil. Working in 2 or 3 batches, blanch the greens for 1 minute (begin counting after the water returns to a boil.) Immediately transfer the greens to the ice water to cool. Drain well, then squeeze out the excess water with your hands. Spread the greens on kitchen towels and pat them dry, as the excess water will cause the oil in the wok to spit. Repeat with the remaining batch(es).
4.Heat a wok over medium-high heat until a drop of water evaporates on contact. Add the oil and swirl to coat the bottom. Add the garlic and stir-fry until aromatic, about 30 seconds. Add the greens and stir-fry for 1 minute. Stir in the oyster sauce mixture, sugar, and optional red pepper flakes. Lower the heat to medium-low and allow the greens to cook for another 3 to 4minutes, stirring occasionally. Season to taste with salt and pepper. Transfer the greens to a plate and serve.