There's no point, they very explicitly don't expect candidates to know OCaml before joining. You can interview in just about any language you want.
Get updated on algorithms and data structures, Google interview style.
I wasn't (having been around for a few decades), and while I still managed to swim, failed to impress.
Plus I am not keen in companies that follow Google hiring practices.
https://blog.janestreet.com/interviewing-at-jane-street/
https://blog.janestreet.com/what-a-jane-street-dev-interview...
https://blog.janestreet.com/jane-street-interview-process-20...