OCaml officially supports Linux on SystemZ servers:
IBM Z (s390x) Linux
But what about the z/OS itself? It is still quite popular in the big corporations and financial services. Are there any efforts to port OCaml there? Especially from the JaneStreet’s point of view.
IBM recently released their own C/C++ compiler suite, and new version is based on the Clang/LLVM it seems. So the port in fact might be easier than before. Moreover, you can send them a suggestion to sponsor this work through BountySource platform.