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. 

Reinstalling OS via DCI manager

Choose the server in the Control Panel and click to Panel to go to DCI manager. 

Click Servers in the Main menu on the left. Then choose the server you need and click Operations. 

Choose Operation type - Install OS from template. Then choose OS and click OK. 

If you tick Inform upon completion you'll get a notification in browser about finished reinstallation.