1.In a small bowl mix together oyster sauce, white pepper, sesame oil, rice wine, sugar, water, 1 tablespoon cornstarch, and fish sauce then set aside.
2.Clean the crab and cut into pieces. Pat dry with paper towels and put into a big bowl. Add 3 tablespoons cornstarch to the bowl and lightly coat the crab pieces with it.
3.Heat up a wok and add 4 tablespoons oil. When the oil is heated, drop the crab pieces and deep fry. As soon as they turn red, dish out, strain the excess oil and set aside.
4.Add back 1 tablespoon of oil from previous frying. Add the ginger and stir-fry until aromatic. Put the crab pieces into the wok and quickly stir a few times.
5.Mix the sauce one more time and add it to the wok. Add the chopped green onions, toss the crab in the wok a few times until well coated with the sauce, dish out and serve immediately.