The Silicon Graphics O2, introduced in 1996, is the successor to the Indy workstation. It comes in two flavours design-wise: the R5000/RM5271/RM7000 and the R10000/R12000. The former has space for two harddrives and the latter only for one, due to the size of the CPU heatsink. It is possible to put a R10000 into some R5k O2's by removing the metal separator between the mainboard and the harddrives, however, on some earlier models the separator is part of the chassis and cannot be removed short of cutting it out with a hacksaw.
IRIX versions 6.3 and 6.5 (up to the latest overlay) are supported on this machine, however, only in 32-bit mode, due to the nature of the O2's internal architecture.
The O2 has only one available graphics option (CRM), which is built-in and cannot be replaced. Media options include the Audio Option, Analog Video Option (AV1), Digital Video Option (AV2) and various PCI64 cards ranging from SCSI, Ethernet and Firewire to FDDI.