In the exts dictionary, "ml" is mapped to "ML". So I think the syntax highlighting wouldn't work quite well for OCaml... And I have found OCaml.ml in the hello_world folder, maybe you meant OCaml in the first place? I'm not sure when loading "ML" in the syntax package what LaTeX would actually give you, but it seems to fail in my case where I tried to convert an OCaml script into a tex file.
Thanks!