Pacific Poison Oak (Toxicodendron diversilobum), is a woody vine or shrub with compound leaves of three leaflets.  Flowers appear in May.
Poison oak leaves and twigs have a surface oil, urushiol, which causes itching and allergic rashes in 4 out of 5 humans, after contact by touch, or even by smoke inhalation.
Remember the wise advise "leaflets of three, let it be."