One approach is accessible from Fletcher Penney's MultiMarkdown site.

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

1. copy Fletcher's Markdown.ooxsl to
   ~/Library/Application Support/OmniOutliner 3/Plug-In
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 ...