There are thousands of OCaml libraries available. Here is a list of the most popular sites where you can find them.
Official OPAM repository is the first place to check. The most high quality and most widely used libraries are provided as OPAM packages.
GitHub is an extremely popular code hosting site and very widely used amongst OCaml programmers. Click the link to find OCaml projects. Remember that many of these libraries are provided as OPAM packages, so you'll have already found them in OPAM. However, there are many more libraries here that people haven't pushed to OPAM for one reason or another.
OCaml Forge is a code hosting site dedicated to OCaml projects. Hundreds of libraries are hosted here. Like GitHub, many of these libraries are in OPAM, but you'll possibly find many here that aren't.
Bitbucket is yet another code hosting site. It is far less used amongst OCaml programmers, but click the link to find those that are.
OASIS-DB is a collection of released OCaml packages. It is an alternative packaging system to obtain OCaml libraries.
Caml Hump is now deprecated. For many years, it was the definitive source to search for OCaml libraries. It is hosted on the old caml.inria.fr, which is being retained because there are many links on the internet pointing to this site, but is no longer maintained. You should not rely on it for up-to-date information.