If you have been exposed, within 2 hours, get some Technu. Use it to wash the oil off your skin. The oil causes the allergic reaction.
It can take days to get the reaction. Also, if you are immune, you can suddenly be sensitive. I geocache with my girlfriend and she wades into PO looking for them while I stay away (I got PI when I was 7) She said she was immune. She did wipe her hands with an alcohol wipe when we got back to the car. About 2 days later, the rash was on the back of her hand, on her cheek and on her side where she was adjusting her pants. luckily she used the wipes before she went to the bathroom. They lasted about 10 days.
Mythbusters looked into PO/PI. They smeared PO on Grant's arm in 3 places and tried to wash off the oil with water, vodka and a "topical cream" (likely Technu). The water and vodka did nothing but the cream worked.