Just to add to the sibling comment. To maintain backwards compatibility, OCaml 5 has both threads and domains.

Threads belong to a domain and only one thread can hold the runtime lock for the domain. This is the same behaviour as in OCaml 4.

With OCaml 5 you can have as many domains as you want though (we recommend no more than you have cores though).

This backwards compatibility decision to separate threads from domains has been very useful and allows to gradually "port" existing code to 5.0: first just fixup C bindings to avoid naked pointers, and then code can safely run on OCaml 5 just as before.

And once a program (and all its dependencies) have removed dependence on global state they can opt-in to multicore by spawning additional domains.

To clear up any misconception, out of the box OCaml will behave like OCaml 4 with a single domain and a "domain lock". Programs currently using multiple threads for concurrency will remain single-core for the time being, as they will need to opt-in to parallelism features. In this sense, adding parallelism to OCaml does not break existing programs, but they still might have to be audited for thread-safety depending on how they want to use parallelism. There is no magic.