Yep!
Unix system programming in OCaml - https://ocaml.github.io/ocamlunix/
Huh, I've never actually heard of using OCaml for system programming. Interesting!
Red Hat's Richard Jones (who is also often commenting on Hacker News) does a lot of system programming in OCaml (mostly VM management-related I think):
Those don't require direct use of C, necessarily - but rather some form of FFI to POSIX and the syscall interface. Hell, if one is writing a systemd replacement, I'd encourage use of OCaml. It'd spare you lots of boilerplate because you're writing relatively high level userspace logic, anyway.