On Hetzner virtual machines, installing and running operating systems that are not officially supported is possible. At the time of writing, the following approach is legal and encouraged by Hetzner.

To install OpenBSD, you need to do the following:

1. Get a VM…
2. Boot into the rescue system.
3. Download the OpenBSD image. wget https://cdn.openbsd.org/pub/OpenBSD/7.4/amd64/install74.img (wget is installed on the rescue system)
4. Write the image to the hard disk dd if=install74.img of=/dev/sda bs=4M REPLACE /dev/sda with your disk
5. Reboot, and you will be greeted with OpenBSD install wizard.