I found this. Don't know if they still make it or sell it.
E. E. Bullard Company, Sausalito, CA, sold an extract of poison oak in the 1970s. Glass vials, about one cc each, five in a box, included in first aid kits for the Bell System. Climbing telephone poles exposed people to it. The instructions with the extract directed one to drink a vial, wait a day, drink another,then drink them monthly. Also, they stressed the point that you must avoid exposure to poison oak for the first month. You only waited two weeks, and of course there are other variables.
The product worked well for me (five vials in four months and I was immune for a few years), but was taken off the market due to some people having a reaction to it.