In a large bowl mix pork, water chestnuts, scallions, garlic, ginger, sesame oil, egg, sesame dressing mix, and oyster sauce. Fill center of each wonton wrapper with 1 teaspoon of meat filling. Gather wrapper up and twist to secure sides. Brush the edges of the wontons with water to help seal. Arrange cabbage leaves on the bottom of a bamboo steamer. Place dumplings about 1-inch apart and steam until the filling is cooked through, about 20 minutes.