The Onyx2 OEM CD-ROM is a SCSI Toshiba unit. The drive is connected to an SCA style connector on the Onyx2 midplane by a flex cable. The flex cable revision is the main consideration when selcting a replacement optical drive.
Some early revisions of the flex cable (revision A) only provide 5V to the CD drive, later revisions (revision B) provide both 5V and 12V to the CD. If you have a revsion A flex cable it is possible to make modifications so there is a 12V feed. There are photos of one members modification in this link (tho I'd suggest using a heavier gauge wire): viewtopic.php?f=3&t=16721530&start=45#p7306712
There's a list of *bootable* SCSI DVD-ROMs in the Octane Hardware Aggregator
that should work in your Onyx2 - but just to be safe take some size measurements first.
Of the DVD-ROMs listed in the Aggregator, I prefer the Pioneer slot-loaders. They support the version of audio-over-SCSI used by IRIX, and they'll boot an IRIX Install DVD (an install DVD is something you'd have to take the time to make
, but no having to swap 10 or 12 CDs around every time you install might make it worth consideration.... or there's always DINA
Wrong side of the ocean, but this eBay seller has six of the Pioneer/HP-C7499B slot-loading DVD drives with an Onyx2-friendly black bezel for $12.99 a pop: http://www.ebay.com/itm/HP-C7499B-C7499 ... 56386239c3