For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_systemsmanagement_ansible/15.5/home:ojkastl_buildservice:Branch_systemsmanagement_ansible.repo
zypper refresh
zypper install oniguruma
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_systemsmanagement_ansible/15.4/home:ojkastl_buildservice:Branch_systemsmanagement_ansible.repo
zypper refresh
zypper install oniguruma