Poison-oak is usually a shrub, though it sometimes becomes a vine several inches in diameter that grows high into the oak trees attached by air-roots. The leaves DO come in threes. They are shiney, without prickers, and the middle leaf has a distinct stalk.

It is harder to identify Poison Oak in the winter, when it loses its leaves and looks like erect bare sticks coming from the ground. 

Poison Oak is highly variable. It varies from shrub to vine. The leaves vary from red to green. It has erect stems, leaves in threes, small greenish flowers, and smooth seeds that are about 1/4 inch across. It is often lush in coastal canyons, but sparse in the mountain woodland. It is deciduous, and often looses its leaves in late summer, leaving it hard to recognize. The erect branches give a clue.