@itsfoss As far as I understand it, ONLYOFFICE seems to use OOXML internally. ODF support seems to be achieved by converting the document to/from OOXML whenever opening or saving an ODF document, which results in compatibility issues.
So, due to this architectural design, it might be more or less impossible for Euro-Office to switch to ODF as the default format.
@elgregor @itsfoss From a standards perspective yes.
But personally I'm not really a fan of #LibreOffice. For instance I think its UI is incredibly outdated (it lacks basic things like aligning objects to each other) and I've followed many discussions around UI improvements and they didn't inspire confidence (many weren't open to change).
There's also #Collabora Office, but at the moment it's rather clunky. E. g. it still saves empty documents if you just open it without ever clicking save.