[Jonathan White] "used Automatic duck for the OMF"
From the admittedly limited tests I have done with AD and FCPX, I'd advise you to be very wary of using it in a real world situation until you've looked into all the issues.
My main concern (among others) is that once you detach an audio clip from its video "parent", AD seems to lose all notion of what it's meant to be.
Maybe this is down to specific problems with my tests, but do check out that this at least will work reliably in the context of your project before going ahead.
Simon Ubsdell
Director/Editor/Writer
http://www.tokyo-uk.com