Rehydrate dried mushrooms in hot water for 30 to 45 minutes. Drain, and slice. Trim all fat off oxtails. Spread on a shallow roasting pan. Roast at 450 degrees F (230 degrees C) for 45 minutes. Drain off fat, reserving about 2 tablespoons. Add 1 cup of water to roasting pan in which oxtails were browned.