Since then many things happened: Nextcloud and OnlyOffice have announced their partnership in November 2018 and meanwhile the desktop version of OnlyOffice also offers the integration of Nextcloud.
Using the menu command “Connect to cloud” you can set up a connection to your own Nextcloud server. After the conection is configured it is available directly when starting OnlyOffice the next time:
If you now open NextCloud, the file management of the Nextcloud server appears where you can open or create files as usual. When opening a file with OnlyOffice from a Nextcloud server where the document server of OnlyOffice is available it behaves like the version in the browser – you can work together with other users on a document and contact them:
Combined with its high compatibility with Microsoft Office documents this solution is a true alternative to Microsoft Office 365.