Export OmniOutliner to Markdown
Luckily there is a Community Plug-In on Github which adds the new export feature.
To install it just create a Plug-Ins directory at this file path and then checkout the plugin:
$ cd ~/Library/Containers/com.omnigroup.OmniOutliner4[.MapAppStore]/Data/Library/Application Support/The Omni Group/OmniOutliner/ $ mkdir Plug-Ins $ cd Plug-Ins $ git clone https://github.com/fletcher/Markdown.ooxsl
Then restart OmniOutliner and now you can export to Markdown !
Thanks Omnigroup Support for giving the hint for this project.