Heat 1 tablespoon oil in a wok and cook sesame seeds 1 minute or until golden. Remove with a slotted spoon and set aside. Add garlic and stir fry for 1 minute until aromatic. Remove the garlic and set aside.
Add the remaining oil to the wok and heat until hot. Add the mushrooms and stir fry 2-3 minutes or until tender.
Add the on choy to the wok and stir fry 2-3 minutes or until just wilted. Add the sesame seeds, garlic, oyster sauce, sesame oil and water and toss to coat the on choy. Serve as a side dish with rice.