This came up in a thread on another topic, but I have found it useful and hard to track down directly through a forum search, so I am bumping it up to become a thread of its own.

Fletcher Penney has written an OO plug-in which makes it possible to export an outline from OmniOutliner to a Scrivener outline.

It is available on his MultiMarkdown site.

Scrivener imports MultiMarkdown formatted outlines, and you can export from OmniOutliner to MultiMarkdown format if you:

1. copy Fletcher's .ooxsl file to
   ~/Application\ Support/The\ Omni\ Group/OmniOutliner/Plug-Ins/fletcher-markdown.ooxsl
2. restart 003
3. choose the MultiMarkdown (text) format from File>Export

Once you have created a Markdown text file version of your outline, you can import it into Scrivener through File>Import>MultiMarkdown file ...