The "Flat OpenDocument" format is an open format that is supported by default in LibreOffice.
To open this document format in OpenOffice.org, you have to install the "OpenDocument-Text-Flat-XML.jar" plug-in. This can be downloaded from the MultiMarkdown downloads page. Then simply open OpenOffice.org, go the the "Tools->XML Filter Settings" menu option, and then choose the "Open Package..." button to import the file you just downloaded.
If you're going to share this document with others, you may wish to save to the native OpenDocument format in case those you're sharing with do not have that plugin installed.