Poison oak, you say...
So first, why is poison oak so poisonous? It's not that it's actually poisonous, but it does contain urushiol, an oil that can irritate the skin, causing rashes or blisters. The frustrating thing about poison oak (well, one of many frustrating things) is that it's hard to know immediately if you've had contact with the plant. It normally takes 24-36 hours for any skin irritation to appear.
You can download this comprehensive PDF on poison oak identification here: The Sure-Fire Poison Oak Poison Ivy Identification System, but my quick 4 step system below has worked for me for avoiding poison oak so far. Remember, I am not a medical professional. This is not medical advice. :)
The four-step system
Step 2: Shiny leaves. Poison oak leaves aren't always shiny, but they can be. They can red red and shiny, green and shiny, or not very shiny at all.
Step 3: To distinguish poison oak from other plants with clusters of three leaves (like blackberry), remember that poison oak leaves have smooth, rounded edges, rather than jagged ones.
Step 4: Poison oak has smooth branches with no thorns. If you're looking at a plant with thorns, it's definitely not poison oak.
While just one of these steps can't help you decide whether a plant is poison oak, combining all four steps together can get you well on your way to preventing a very un-fun rash.
So what if you've been exposed?
On official trails, you are unlikely to come into direct contact with poison oak. If you think you have been exposed, wash your clothes and your skin as soon as you can. Products like Tecnu are also handy to have around as they can remove poison oak oil from your skin.
Stay safe out there, urban hikers! Now you can impress others with your poison oak identifying skills.