Rocq is an excellent example of something OCaml was designed for. FFTW3 is another great example. Unison too.
Generally, you want stuff where you have to build a fairly large core from scratch. Most programs out there doesn't really fit that too well nowadays. We tend to glue things more than write from nothing.
Rocq, Pfff and Flow, Vpnkit in Docker for Mac and Windows, the web version of Facebook Messenger at a point, Tezos, Unison, the Xen cloud platform, the Mirage unikernel.
What applications are written in OCaml? All I can think of (which says more about me than it does about OCaml) is the original Rust compiler.
Even Haskell has Pandoc and Xmonad.