[bonyt]

Looks like openDesk uses Collabora Online, which is itself based on libreoffice online - web based libreoffice.

https://www.opendesk.eu/en/product#document-management ("Collabora Online powers openDesk with a robust office suite designed for efficient teamwork and secure document editing.")

https://en.wikipedia.org/wiki/Collabora_Online ("Collabora Online (often abbreviated as COOL) is an open-source online office suite developed by Collabora, based on LibreOffice Online, the web-based edition of the LibreOffice office suite.")

[trelane]

More than that--Collabora is a major (maybe the biggest) contributor to LibreOffice.