For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:ocaml/openSUSE_Tumbleweed/home:frispete:ocaml.repo
zypper refresh
zypper install ocaml-jsonm
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:ocaml/15.4/home:frispete:ocaml.repo
zypper refresh
zypper install ocaml-jsonm
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:frispete:ocaml/15.3/home:frispete:ocaml.repo
zypper refresh
zypper install ocaml-jsonm