Poison Oak isn't a problem underneath the main cliff band - I've only ever seen one sprout of it underneath one of the small blocks at the base, and it had gone the next time I went (a few weeks later).
That said there is quite a bit of Poison Oak elsewhere in the park, including around the top of the "Unnatural Act" area, so it's worth watching out for if you get off the beaten track.
Although Glen Park often gets billed as "the only, and therefore the best, bouldering in San Francisco", this description doesn't do the place justice at all. In reality Glen Park is a great little bouldering area that offers a perfect introduction for beginners (good rock, lots of classic easy & moderate highball problems), while offering virtually limitless harder eliminate problems for those with a bit of imagination.
It is a fun place, though I haven't been back there in a few years now. The only major CON of the place is poison oak. Especially in the winter season when the leaves are gone but the branches still totally alive. I was covered head to toe with "the itchies" after a winter session here back in '97. I still think it's fun just watch what your touching other than the rock.