What operating system is installed on my VPS?
You determine the operating system for VPS already during the ordering process, where you can choose the installation image from which you will go through the manual VPS installation process via the remote screen. The only exception are pre-installed solutions such as WebApps server and MS Windows 2012 R2 in the application section, where installation will take place without the need for your intervention.
If you change your mind later, you can reinstall your VPS via the user interface at any time. Note, however, that any data stored on your server will be lost when you reinstall the operating system.