HN user
Opium isn't mature but it's mostly a friendly wrapper for an http layer that is fairly mature.
Have a look at: https://github.com/mirage/ocaml-cohttp
You can of course also use cohttp directly but it's a little raw.
I think the GP wanted something with js_of_ocaml. Nevertheless, as the author of Opium, I am grateful for your recommendation :)
If you want something cohttp based https://github.com/rgrinberg/opium might serve your needs.
"but I can't think of anything it does that most other programming languages don't do better."
What do you mean by that? Io is slow and does not have the extensive libraries that are present in other languages. However, as a pure prototype oo language, it is second to none.
Read here: http://c2.com/cgi/wiki?IoLanguage