[Irinikus]

As far as I know, for machines such as the O2, the PROM is updated automatically to the latest revision associated with the version of IRIX you're installing, when you install IRIX.

For a machine like the Tezro on the other hand, it's a totally different story, as the PROM on such a machine is not updated automatically when you install IRIX, and therefore needs to be flashed manually. (There is a trick to doing this on a tezro, which involves the installation of previous versions of IRIX to update the PROM revision progressively, to avoid the possibility of maybe bricking the machine.)

On the O2, this is not a problem as the PROM revision is updated automatically with a new IRIX install.

Therefore if you buy a disk with IRIX preinstalled, the PROM on your O2 won't be updated automatically.