For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:illuusio/openSUSE_Slowroll/home:illuusio.repo
zypper refresh
zypper install doxygen
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:illuusio/16.0/home:illuusio.repo
zypper refresh
zypper install doxygen
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:illuusio/15.6/home:illuusio.repo
zypper refresh
zypper install doxygen