Drain the tofu well in paper towels, then slice 1/4 inch thick. Keeping white and green parts separate, cut the scallions into 1/2- to 1-inch pieces. Heat oil in a wok or heavy skillet. Over high heat, sauté tofu slices on one side for 1 to 2 minutes. Add white parts of scallions and cook several minutes longer, until tofu is browned on the bottom. Using a wide spatula, turn over and cook on the other side for several minutes. Add green scallion tops, then cover skillet and steam for 1 to 2 minutes. Remove to a serving dish and drizzle with oyster sauce. Serve immediately.