Several different manufacturers offer magneto-optical (MO) drives and disks, ranging in capacity from 128 MB to 9.1 GB. Some of these are write-once like CD-Rs (the acronym WORM, for Write Once, Read Many, applies to such disks and drives); others are rewritable like CD-RWs and can be erased. The primary advantage of MO technology over CDs and DVDs is longevity: MO media is typically rated for long-term archival storage on the order of 100 years. On the other hand, MO media is extremely expensive, as are the drives themselves. The mechanisms are considerably slower than conventional optical drives. And MO media comes in many different formats and sizesonce you choose a media type, your future options may be limited.
The latest development in MO is called UDO (Ultra-Density Optical), with disks that can hold as much as 30 GB each. If regular MO drives and media are expensive, UDO is out of this world: plan on spending about $4000 for a low-end drive, plus upwards of $60 for a single rewritable cartridge. Because the backup plan I'm recommending here does not require extremely long-term storage of media, and because I assume you do not wish to spend more on your backup device and media than what you paid for your Mac, I see no reason to consider MO or UDO drives.