Base system in BSD isn't "immutable" per say; the filesystem is mounted rw and is prone to the regular unix file modes. The base system is more just the userland that was written by the OpenBSD project themselves (plus some 3rd party components that are dependencies like perl and clang), which typically isn't on Linux, as most Linux distributions simply use GNU userland or similar; so everything is 3rd party.
That being said, it is very easy to replace the base system should anything go wrong, simply by re-updating to the same version inside of bsd.rd on OpenBSD.