Whisk together the oyster sauce, soy sauce, rice vinegar, sesame oil and cornstarch. Set aside.
In a large skillet, warm the oil over med-high heat. Add the chicken and cook, stirring or tossing frequently, until it is cooked through, about 6 minutes. Remove the chicken to a plate and set aside.
Add the zucchini to the empty skillet. Cook, stirring or tossing frequently until the zucchini begins to soften, about 3 minutes. Add the garlic and cook, while tossing constantly, for an additional minute until fragrant.
Add water to the pan and use a wooden spoon or metal spatula to scrape up all the browned bits. Pour in the soy sauce mixture and return the chicken to the pan.
Cook, while stirring constantly, until the sauce thickens and coats the chicken, about 1 minute. Sprinkle with green onions and sesame seeds. Serve over rice if desired.