Wondering if it's bad to take a hot shower if you have poison oak?

Maybe/maybe not. I can see hot water causing an initial increase in blood flow at your skin surface & intensifying any discomfort/reaction to the rheus antigen. The shower itself is a good idea, since lathering up well with soap should remove the rash triggering chemicals. I would use warm water instead.
Should be fine . Poison oak causes contact dermatitis at the area that you expose to it. A hot shower should not spread it. Try using otc Hydrocortisone cream and moisturizer. Seek local medical help if symptoms are severe.