There are also Fibre Channel adapters for these if you have the XIO-PCI card cage. Can boot from it with some special magic IIRC, but a local SCSI drive is fine for most things.
It might have actually been Adaptec PCI in an SGI XIO-PCI card cage, memory is failing me. Definitely some dark magic required on Irix and on the FC switch and storage target side.
So after my XIO FC board died, I added an Adaptec board to my O2K to get my RAID array back online (the whole company was out of work because of that failure).
However though the card appeared in "hinv", it didn't work... After I asked for help on the newsgroups (the system was out of maintenance...) Alexis sent me a private email with the necessary invocation of the kernel rebuild command, and it worked like a charm from there.
https://store.inertialcomputing.com/product-p/scsi2sd-v5-st3...
But sure, if you're just tooling around, nfsroot is convenient.
Though network booting and storage may well be an easier avenue and cheaper. If you already have a NAS, that may well be the easiest path.
It's possible to install gigE, but you will need a PCI-X shoebox and also a NIC with compatible firmware. Not super easy to find.
You will want gigE if you are going to do anything interesting over the network on an Octane.