Clean the oxtail and beef thoroughly. In a stockpot, boil the oxtail and beef in 6 cups (1½ liters) of the water for about 10 minutes. Drain and discard the water. Pour the remaining water into the stockpot. Bring to a boil then simmer over low heat until meat is tender, about 1½ hours.
Remove the meat from the pot, set aside and reserve the stock.