You can choose one of the following operating systems from the list while ordering:
CentOS 7-amd64
Debian 8-x86_64
Debian 10-x86_64
FreeBSD 11-amd64
FreeBSD 12-amd64
Ubuntu 16.04-amd64
Ubuntu 18.04-amd64
Ubuntu 20.04-amd64
You can reinstall OS via DCI manager any time.All OS from the list above you can install for free, also you can choose Windows Server 2012, Windows Server 2016 or Windows Server 2019 for the additional cost while ordering server.