Heat oil in a wok or large heavy skillet. Add chicken, garlic and oyster sauce, and stir-fry for 10 minutes. Stir in broccoli, green pepper, carrots, cabbage, celery, bean sprouts, zucchini and green onions. Season with salt, and stir-fry for 6 to 8 minutes. In a small bowl, mix together water, soy sauce and cornstarch. Stir into vegetables, and cook for 1 to 2 minutes, or until sauce is thickened.