I've been eradicating poison oak for over 30 years, and still have enough to go another decade I'm sure.
Roundup or Ortho (can't remember which Ortho... it says 'for poison oak' on the bottle) are used to spray every spring. Some years I'll spray 50 gallons (mixed); other years maybe only 16 gallons. I've beat it back well in excess of 400 ft from the house, most places more. Each year I spray as far as the stream will reach, then the next year I can get that much farther into it for that year's spray activity.
On the vines up the trees, I use loppers to sever it, then coat the 'stump' with whatever juice I'm spraying that year. Usually there'll be a bunch of small shoots from the vines root system the next year, but it gets sprayed once and is gone.
On my "spray days" I'll wander all the cleared areas and usually find a few little sprouts to attack.
My biggest recommendation is to get a backpack sprayer, unless you have a powered tractor mounted sprayer and can access it all by tractor. Our manzanita is so thick that no vehicle could get to the places I spray. Sometimes I'm crawling on my hands and knees with the sprayer.
Phil
