Ondemand server supporting clusters with different OS?

It can, really it doesn’t matter so much to OOD itself.

What you’ll have issues with is the apps. Specifically VNC apps that use XFCE - See these topics below. We’ve made some headway on this issue (both myself and the original posters) - but I don’t think it’s 100% resolved.

Basically the TLDR is that XFCE updated from 12 to 14 and has some compatibility issues with the way all our scripts are written.

We haven’t updated our own apps, so it’s not 100% solved but we’ve at least reduced the problem space a bit. In any case - you’ll likely have to modify some of the VNC app scripts so that they can recognize whether they’re on REHL 7 or 8 and act accordingly.