Potatoes contain natural astringent and antimicrobial compounds that soothe the rash caused by coming into contact with poison oak. These compounds also help to reduce swelling and infection. Thoroughly wash a medium sized potato, then finely grate or chop and mash into a smooth paste. Apply the paste to the affected area, being careful not to rub too hard and further aggravate the skin. Leave until dry, then rinse off and apply more. If you have plastic wrap handy, spread the potato on a piece and wrap the affected area. Repeat every few hours until you experience relief.