Western Poison Oak (Toxicodendron diversilobum) is a native deciduous plant that lives throughout most of California, and in particular, along riparian areas. It is a plant that hikers often encounter and should know how to identify. It is quite common in Upper Bidwell Park, so if you are out and about up there, be watchful that you don't stumble through it. The berries that the plant develops during Spring are an important food source for local wildlife, in particular, birds. Unfortunately, the oil urushiol is exposed on the plant's surfaces and are particularly nasty when they come into contact with human skin. If you are not allergic now, you may be in the future with repeat exposures.
Poison Oak is described as having sets of three leaflets (as shown in the photo) with a waxy texture. If the summer is particularly hot, poison oak may begin the autumnal process in order to conserve water and nutrients. The leaves begin turning shades of red. Remember: Leaves of three, let them be!
Poison Oak can be found in riparian areas, pine and oak forests, and other random places. Birds excrete their seeds after consuming berries and so will grow in sometimes seemingly random places. However, you can be assured to run across Poison Oak in Upper Bidwell Park and the Mendocino National Forest. Always be on the look out... plan your route several feet ahead!
Avoid Poison Oak if at all possible. Should you encounter it, wash using cold water so as to prevent your skin pores from opening up and sucking in the oils. If you know you are particularly prone to poison oak rash, consider making a trip to your nearest drugstore for some Tecnu or other soap specifically for preventing or minimizing such outbreaks. These topicals are specifically designed to break down the Urushiol oils. If you find that you have a poison oak rash, antihistamines and calamine lotion are your best bet to relieve the itching. It can take several weeks for it to clear up once you have developed a rash, so your best bet is obviously to just avoid it whenever possible. Of course, nothing replaces the advice of a doctor.
If clothing comes in contact, wash separately in hot water. Adding a soap specifically made to break down the Urushiol oil to the wash cycle may be advisable.
Interestingly, some people have no reaction to Urushiol, meaning that they have no negative reaction to poison oak, poison ivy, etc. It tends to run in families. Consider yourself lucky, and every once in awhile you might get a chance to help clear out a friend's yard.