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I'm guessing at least the sponsors of the OCaml software foundation: https://ocaml-sf.org/sponsors/

Plus Facebook with their Flow[1], Infer[2][3], Airbus with their BinCAT[4], and Frama-C[5].

[4] https://github.com/airbus-seclab/bincat

I work at Bloomberg in derivatives. C++ is our backbone but there is definitely a lot of OCaml in production. To tell it all we have BLAN, an OCaml-ish language that you (as our customer) can use to structure (exotic) derivative contracts and get them priced in DLIB.

I know that Ahrefs and OneGraph use it for production.