Heat oil in large frypan over medium heat.
Add scallions, ginger and garlic.
Stir-fry until scallions are softened, about 3 - 5 minutes.
Add half of the white mushrooms and the shiitake mushrooms; stir-fry until almost all liquid is evaporated, about 7 - 10 minutes.
Remove to a plate and keep warm.
Place remaining white mushrooms in fry pan and stir-fry about 5 minutes until most of liquid is evaporated.
Add oyster mushrooms and stir-fry 3 minutes longer.
Return cooked mushrooms to frypan.
Add soy sauce and brown sugar; stir until heated through.
Garnish with orange zest, if desired.