Poison Ivy and poison oak are very similar in different areas to the point that you can easily conclude that there is no difference between the two. The only difference is that they are two plants entirely. Poison ivy and poison oak belong to genus Toxicodendron. Both contain a substance known as urushiol.
The effect of this substance seems more like a poison due to the fact that it has the ability to cause blistering or itching rashes for most people. As soon as urushiol begins to react with the skin, it causes the irritation of the skin as well as swelling. Depending on the nature of the skin, anybody that touches any of the two plants may have certain skin diseases like dermatitis i.e., the inflammation of the skin.
Poison oak and poison ivy can only cause itching rashes only on areas where you have the substance your skin as it cannot spread to other areas, except if you have the substance in your hand and you touch other parts of your body unknowingly or by mistake.