It is not the only one though. There are Ahrefs, Tarides, Tezos, Citrix (XenServer and a part of Xen are in OCaml), Inria, Facebook (for compilers, typecheckers

It is not the only one though. There are Ahrefs, Tarides, Tezos, Citrix (XenServer and a part of Xen are in OCaml), Inria, Facebook (for compilers, typecheckers and ReasonML), Bloomberg (was Bucklescript/ReScript, now at facebook though I believe). There are also some academic projects with industrial uses. Directly to mind come Coq, Frama-C, Mirage and the Zélus compiler. EDIT: added Inria and Frama-C