For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Base:System/openSUSE_Tumbleweed/Base:System.repo
zypper refresh
zypper install btop
For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Base:System/openSUSE_Factory/Base:System.repo
zypper refresh
zypper install btop
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Base:System/15.5/Base:System.repo
zypper refresh
zypper install btop