Woah!!! After hours of trying out this, that and the other your post put me on the path to a solution which works on a Synology NAS. Thank you timo12357!
On a Synology the web base directory is /var/services/web and the nextcloud installation is /var/services/web/nextcloud. So I used your example to do:
ln -s /var/services/web/nextcloud/ocm-provider/ /var/services/web
ln -s /var/services/web/nextcloud/ocs-provider/ /var/services/web
It worked! I rebooted the NAS to make sure the links survived a reboot and all good.
This is a working solution for those of us using a Synology NAS folks!