I finally made OPML to run behind OS X Server 5.
It is a bit different to Dave's solution from back then http://scripting.com/stories/2008/11/19/checklistForReverseProxies.html
Install OPML and OS X Server 5.
Set up your website - just like to two default entries for ports 80 and 443 (SSL), choose your servers IP address (e.g. 126.96.36.199).
Then add the following
ProxyPass / http://127.0.0.1:5337/
ProxyPassReverse / http://127.0.0.1:5337/
and delete the entries
DirectoryIndex index.html index.php default.html
I tried other ways, but only this worked. In Server 5 everything runs behind a proxy and the config files are located /Library/Server/Web/... So don't bother with the files in /private/etc/apache2/ or the httpd-vhosts.conf in the /extra, because the do not affect the Server.app Apache instance. /Library/Server/Web/Config/apache2/ReadMe.txt has a few infos on the why.
There is another way to integrate applications, but that looked way to complicated for me.