For openSUSE Leap 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:haxe/openSUSE_Leap_15.6/devel:languages:haxe.repo
zypper refresh
zypper install opam
For openSUSE Leap 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:haxe/openSUSE_Leap_15.5/devel:languages:haxe.repo
zypper refresh
zypper install opam
For openSUSE Leap 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:haxe/openSUSE_Leap_15.4/devel:languages:haxe.repo
zypper refresh
zypper install opam