After rinsing, not scrubbing, your skin with hot water to rinse off the oil do the following to relieve the itch. Draw a bath and add 1/2 cup of fresh baking soda (not the box that has been deodorizing your fridge) and soak for a 30 minutes. Once out of the bath, using a three to one ratio of baking soda to water, make a paste to apply to the affected area. Let the paste dry on your skin — it is believed that the baking soda pulls the caustic urushiol away from the skin, decreasing the itchiness.
If you experience a severe reaction after contact with one of these rash-producing plants, seek medical treatment. Prescription medication might be needed to reduce the swelling and itch.