How long does it take for poison oak rash to appear on the skin?

Not immediate. The reaction to poison ivy and poison oak is called "delayed hypersensitivity" because it usually takes 48-72 hours for a rash to start after the exposure.
Poison ivy. It may take 3-10 days to develop a rash from poison ivy/oak/sumac the first time. However with each exposure the reaction becomes faster and stronger because the body stores white blood cells called "memory cells" to be more prepared for the allergy next time. With repeated exposures, the reactions can become quite severe and may develop within a day.

Related Questions

How can I tell if my child’s skin rash is from exposure to poison oak?

Contact dermatitis. Poison ivy, oak and sumac all contain the same substance which causes a severe contact skin reaction in most people. The rash will appear from about 24-48 hours after the exposure to the plant oil. It is red, raised, very itchy and often has a seepy discharge or blisters. It will often appear in "streaky" lines on the areas which were exposed. Rash on the face can cause a lot of swelling. Read more...
Streaks, blisters. The allergic rash can cause itching, burning and blistering 1-2 days after contact with poison oak. The blisters may ooze clear fluid and may be different sizes. As they heal, they may look crusty. The rash may also look like pimples. Read more...