Bring 4 cups water to a boil in a large Dutch oven. Add oysters; cook 3 minutes. Drain oysters, and set aside.
Melt butter in a large skillet over medium-high heat. Add garlic, onion, and lemon grass; sauté 2 minutes. Stir in celery and next 8 ingredients; reduce to medium heat, and cook 6 minutes or until peppers are tender. Stir in oysters, green onions, and cilantro; cook 1 minute or until thoroughly heated. Serve over rice.