If you don't have oyster sauce, you can use hoisin sauce or even BBQ sauce. All will lend a savory sweetness as well as give a little body to the sauce.If making for vegetarians, make sure to use a vegetarian oyster sauce or other vegetarian sauce (regular oyster sauce is not vegetarian.)
In bowl combine the garlic, sesame seed oil, soy sauce, rice vinegar, oyster sauce, water. Set aside.
Heat large skillet on medium heat. Add the oil, then the green beans. Saute for about 2-4 minutes, stirring occasionally, or until nearly tender (timing will depend on size & freshness of green beans, and preferred texture.)
Stir in the sauce mixture and cook for 1-2 minutes, or until sauce thickens slightly and coats the green beans nicely.Season with black pepper and salt, if needed, to taste. Finish with any desired toppings. Serve warm.