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Facebook's static analyzer Infer https://fbinfer.com is written in OCaml.

There's a lot of OCaml in the program verification space: https://frama-c.com http://why3.lri.fr https://alt-ergo.ocamlpro.com

And for mysterious reasons, OCaml is now kinda popular for.. web frontends. Bloomberg created an OCaml-to-JS compiler https://bucklescript.github.io and Facebook (again!) created an alternative syntax https://reasonml.github.io and this combination is apparently a new hipster way of writing web apps.