I finally got a OAC yesterday. What helped me turn the corner from lots of two hand up, one hand negatives, was strap assisted chins on the extreme rings. Its similar to the assisted chins in Coach's book, but you use the hanging part of the strap instead of the vertical support. I just started doing chins with one arm holding on to the excess strap hanging from the other ring. Gradually, I worked the chin ups where I was grabbing the assist side strap lighter and moving it lower. Last night I was able to get a legitimate OAC on the right. I'm almost there on the left but I still need some assist by pulling on the strap.
Good luck. Oh. and eating less crap helped my strength to weight ratio as well.