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.