The bottom of janestreet.com has a “Code Library” link that points to https://janestreet.github.io/, but it looks like that page is either being deprecated in favor of this new site or perhaps is just out of date. It might be good to either replace that link, or add a link to this new site, depending.
It would be nice to have some way of navigating to different versions of the documentation. So far as I can tell, if I go to https://opensource.janestreet.com/base/ and click the “API Docs link” there is no sequence of links I can click that will get me to an earlier version of the library. I either have to guess the URL scheme or chop the URL down to https://ocaml.janestreet.com/ocaml-core/ and navigate from there. Possibly this is a limitation of the documentation generator.
In a similar vein, it maybe would be useful for the documentation page itself to have a link to the changelog.