A bit of east-west fusion here. Oyster sauce is available in the Asian section of grocery stores.
2 T water
1 T soy sauce
1 T oyster sauce
2 T vegetable oil
1 large bunch bok choi, trimmed and cut crosswise into slices
1/2 t salt
2 T unsalted butter
1. Stir together water, soy sauce and oyster sauce in a small bowl.
2. Heat the oil in a large, heavy skillet over moderately high heat until hot but not smoking. Add bok choi and salt and stir-fry for about 2 minutes. Add the water soy oyster mixture and butter and continue stir-frying until bok choi is crisp-tender, another couple of minutes.