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.

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).