Heat the olive oil in a large skillet or wok until the oil shimmers, then toss the green onion and garlic in the hot oil until fragrant, about 20 seconds; stir in the mushrooms. Cook and stir until the mushrooms begin to shrink slightly. Sprinkle the mushrooms lightly with salt. Continue to cook the mushrooms, stirring often, until they are tender, about 5 minutes. Pour in the oyster sauce mixture, then stir until the sauce is thickened and coats the mushrooms, 30 seconds to 1 minute. Pour the thickened mushroom sauce over the bok choy; serve immediately.