Heat the peanut oil in a wok or large skillet over medium-high heat. Add the garlic and stir-fry for 20 seconds. Add the green beans and stir-fry until they begin to soften, about 5 minutes.
Stir in the oyster sauce, soy sauce, and sugar. Reduce heat to medium. Continue cooking for several minutes until the green beans have become as tender as you like them.