Heat oil in a pan over medium heat. Sauté garlic until golden, then add onion and cook until soft.
Add the ground pork and cook until browned.
Stir in the oyster sauce (if using), soy sauce, vinegar, and water. Bring to a boil.
Add the string beans. Cover and simmer for 5-7 minutes, or until beans are tender but still crisp.
Season with salt and pepper to taste. Serve hot with rice.