Place mushrooms in bowl and cover with boiling water. Let soak 20 to 25 minutes, until tender. Drain, rinse and reserve.
Mix together oyster sauce and dark soy sauce. Set aside.
Heat wok until smoking hot. Add 1 tablespoon oil, then beef. Stir-fry until meat is browned all over. Transfer beef to plate, and carefully wipe out wok.
Add garlic, ginger, mushrooms and bok choy, and stir-fry for 2 to 3 minutes, until greens are softened. Add steak and sauce mixture. Cook until warmed through and slightly thickened, about 3 minutes more.