For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:alois:branches:Virtualization/openSUSE_Tumbleweed/home:alois:branches:Virtualization.repo
zypper refresh
zypper install virtualbox
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:alois:branches:Virtualization/16.0/home:alois:branches:Virtualization.repo
zypper refresh
zypper install virtualbox
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:alois:branches:Virtualization/15.6/home:alois:branches:Virtualization.repo
zypper refresh
zypper install virtualbox