Are there any options in LibreOffice to save files directly in Google Drive by default? Other than Oog2google extension.
I am unaware of any such option. However, a simple way to accomplish the same objective is to install InSync (more info and download/installation instructions on this page) and save documents directly from LibreOffice into your new '/home/InSync' folder. When you run InSync, any files in this folder will be uploaded to your Google Drive.
OpenOffice plugin ooo2g works on Libre Office too and might be what you're looking for.
To install, first of all you need to get the libreoffice java common package
Then you can download the extension from the official site
And finally install it from libreoffice, opening
Now close and re-open libreoffice and you will se a new floating menu, which is able to export your document directly to GoogleDocs