The OCaml website lists some companies it [0], I think there are a bunch of new blockchain startups that are also using OCaml but are not on that page.
Another might say it was a weakness for snake oil.
To give you an example of how you would use some crypto-related library that implemented its API (?) in OOP:
let x = new Foo.bar in
x#absorb a;
x#squeeze b;
x#reset
This (this entire file) is a great example, too!