Heat a wok almost to smoking and swirl in the oil. Add the garlic, scallion and ginger and stir-fry for 10 seconds. Add fiddleheads (drained, if using canned); stir-fry for 30 seconds. Add hot water and cook the ferns for 1 to 2 minutes, or until still crisp yet tender. Stir in soy sauce and oyster sauce and cook for 20 seconds. Correct seasonings, adding more oyster sauce if desired. Serve at once.