For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Virtualization/openSUSE_Tumbleweed/Virtualization.repo
zypper refresh
zypper install mkosi
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Virtualization/openSUSE_Slowroll/Virtualization.repo
zypper refresh
zypper install mkosi
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Virtualization/16.0/Virtualization.repo
zypper refresh
zypper install mkosi