Info:
Do not install the Document Server
There are two ONLYOFFICE apps - Community Document Server and ONLYOFFICE. Be sure to install the latter.

To integrate ONLYOFFICE into Nextcloud for office document editing and collaboration, install ONLYOFFICE from the Nextcloud app library and configure the plugin as follows, adjusting the domain and secret: (example)

Document Editing Service address: https://office.domain.com

Secrete key: changeme

If your Nextcloud has a self-signed certificate, you should also add the following stanza under services -> CoAuthoring in /app/data/config/production-linux.json using the File manager. Be sure to restart the app after adding the section below:
  "services": {
    "CoAuthoring": {
      "requestDefaults": {
        "rejectUnauthorized": false
      },
      ...
    }
  }