You can stop itching from a poison oak by taking a cool shower, the longer you shower, the better chances you have of reducing the rash. You can also apply some natural toothpaste that has clay or peppermint ingredients. You can get more information at the voices.yahoo.com/stop-itching-poison-ivy-poison-oak-naturally-56164.html?cat=5 website.
The itching is caused from an oily resin in the plant called urushinol. Here's the recommended treatment from the Mayo Clinic:
Apply an over-the-counter corticosteroid cream for the first few days. Apply calamine lotion. Take oral antihistamines, such as diphenhydramine (Benadryl, others), which may also help you sleep better. Soak in a cool-water bath containing a colloidal oatmeal product (Aveeno) or baking soda. Place cool, wet compresses on the affected area for 15 to 30 minutes several times a day.
Ok this may sound funny but take a bath in oatmeal i got poison ivy once and it itched sooo bad but my mom told me that when she was younger her mom made her take a bath in oatmeal and it got rid of the itch and it helped me so much :)