I've been trying to progress to a OAC or MU for a long time, but nothing has really been working for me. I just thought up an exercise that might help, but I'm not sure if it's worth devoting a lot of time to it. Basically I've got my XR hanging about 1.5 feet from my pullup bar, and I put one hand over the bar and the other on the rings (possibly with a false grip). Then I just pull as hard as I can, and then start pushing with the ring hand, until I get to the top. I figure I can lower the XR to emphasize the OAC, or raise it to emphasize the MU.
Sort of like the assisted OAC, but I can put my assisting hand much lower and still do the exercise due to being able to push off it. Its much harder than a regular chin-up (I can do 14 chin-ups, but only 2 like this). Worth doing?