The itching and scratching of poison oak is caused by urushiol oil, excreted by both the oak and its cousins poison ivy and poison sumac. Almost everyone is allergic to urushiol oil; when it gets on the skin, it causes itching, redness, swelling and terrible blisters. Scratching at a poison oak rash only makes things worse, because it spreads the oil further across the skin (including your fingers--which means it will spread to any patch of skin you touch). Poison oak rashes tend to appear in lines, corresponding to the place where you brushed against the plant. That's why it's important to keep your arms and legs covered when moving through areas that contain poison oak, and to wash any clothing you wear--with hot water and detergent--as soon as you possibly can.
The healing period for poison oak varies depending on the extent of exposure and any complications which may develop. In most cases, the rash starts to appear within 48 hours. It typically lasts from one to two weeks, but can last up to three weeks or longer in some cases. When the rash is severe, it may spread further after a few days. Some people believe that such spreading is caused by fluid oozing from popped blisters, but it's probably due to secondary exposure from scratching or similar circumstances. If the rash lasts longer than three weeks and still isn't healing, contact a doctor. You should also contact a doctor if you develop a fever, if your blisters ooze pus instead of clear fluid or if the rash affects sensitive parts of the body like your eyes or mouth.
Treating poison oak has a bearing on how long the rash and itching go on. The aim is to control the symptoms: Reduce the urge to scratch the itch and keep the redness and swelling from becoming a problem until they have time to heal. The best nonprescription medications include corticosteroid creams (like hydrocortisone), aloe vera gel, calamine lotion or anything with menthol as an ingredient. They soothe the skin and (in the case of corticosteroids) help address the allergy directly. In some cases, oral antihistamines can be helpful as well. In the worst cases, scratching can cause a bacterial infection, which requires antibiotics and a doctor's care to properly treat. You may also wish to get prescription-strength corticosteroids if your doctor feels it is warranted. If the rash isn't so bad, you can use home remedies such as cold compresses or colloidal oatmeal baths to help reduce the symptoms.