This is a tough move that some of the Bar-barians (like Zef) can do very easily:
I have been working on it for a while and I still don't quite have it. I figured I just wasn't strong enough to do it. I think it might partially be a problem of technique though. If you watch Zef you will see that he brings the shoulder of the non-working arm towards the bar (Jim Bathhurst also mentions that he generally turns towards the bar while doing the OAP in his excellent OAP tutorial ). I have been trying to pull straight up and getting stuck about half-way. This makes sense because at the half-way point your elbow is at a 90 angle and your body is at it's furthest from the bar. By turning towards the bar you reduce your distance, and increase your mechanical advantage.
For the last couple of days I've been thinking about this as often as 16 y.o. boys think about sex. I'm obsessed. Been itching to try it but I'm still too sore. Hopefully, tomorrow I will be recovered and I will make some further progress implementing this technique.
If you have been working the OAP, let us know how you are doing with it. Successes, or failures, I'm interested to hear peoples' experiences.