Jane Street, a quantitative trading firm with offices in London, New York, Hong Kong, and Amsterdam, chose OCaml because its rich type system improves the quality of their code and catches bugs early.
Jane Street is the largest commercial user of OCaml, using it for everything from research tools to trading systems to systems infrastructure to accounting systems. Software built in OCaml helps them safely trade billions of dollars each day. OCaml's powerful static analysis tools help enforce important invariants so that their programmers can focus on quickly adapting to ever-changing market conditions.
Jane Street has over five hundred OCaml programmers and over 30 million lines of OCaml. Nearly a million lines of their code are open source, and they've created key parts of the open-source OCaml ecosystem, like Dune, the community's de facto build system; Core, an alternative standard library; and Async, a cooperative concurrency library. You can learn more by checking out their tech blog.
For the past 20+ years, Jane Street has used OCaml as their single-tool solution. When deciding on a programming language, they wanted something that was fast yet efficient, caught bugs early, and that they could use both in critical trading systems and in everyday utilities.
Jane Street found that OCaml gave them the best of compiled and dynamic languages. Thanks to an expressive type system with powerful inference, it's concise, safe, and performant. Large codebases built in dynamic languages can quickly become unmanageable; OCaml's type system, deeply integrated into the editor, makes Jane Street's large software systems more legible and easier to maintain. As an added bonus, OCaml tends to attract some of the best programmers, and leads interns to want to join the company full time; using OCaml has been a boon for tech recruiting.
In short, OCaml helps Jane Street quickly adapt to changing market conditions, and move smoothly from prototypes to production systems.