ZFSOnLinux is a kernel module and not a user space application (and you cannot OOM kill kernel threads) and the overcommit probably isn't direct, e.g. there are 64 worker threads who all need some memory to do the work and tell the kernel that memory allocation must not fail. If the memory isn't available either Linux starts killing userspace processes or gets stuck. The same thing can happen in Solaris except there ZFS doesn't use a separate memory mechanism which does not properly react to memory pressure.