Do you mean this tool, or OCaml? They are definitely writing their actual trading systems in OCaml, they have been publicizing this for many years.

that is what i thought. the actual trading code is probably in c or c++ or is in an fpga being controlled by the trading code.

That's largely what their excellent Signals & Threads podcast is about.

https://signalsandthreads.com/

> Listen in on Jane Street’s Ron Minsky as he has conversations with engineers working on everything from clock synchronization to reliable multicast, build systems to reconfigurable hardware. Get a peek at how Jane Street approaches problems, and how those ideas relate to tech more broadly.