For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:javierllorente/openSUSE_Tumbleweed/home:javierllorente.repo
zypper refresh
zypper install apache2-icons-oxygen
For openSUSE Current run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:javierllorente/openSUSE_Current/home:javierllorente.repo
zypper refresh
zypper install apache2-icons-oxygen
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:javierllorente/15.4/home:javierllorente.repo
zypper refresh
zypper install apache2-icons-oxygen