In a small bowl, stir together sherry, water, oyster sauce, and cornstarch; set aside.
Heat oil in an 8- to 10-quart pan over high heat.
Add ginger and white parts of onions; cook, stirring, just until onions begin to brown (about 2 minutes).
Add sherry mixture and bring to a boil, stirring constantly (about 1 minute).
Add crab pieces and green parts of onions; stir to coat with sauce.
Reduce heat to low, cover, and cook, stirring occasionally, just until crab is heated through (5 to 8 minutes).
Mound crab pieces on a warm large platter.