The MO disk ("Magneto-optical") is a form of information disk used by Umbrella. The disk-type, which uses lasers and electromagnets to read its recording layer, is common in Japan, though has fallen into disuse in Western nations. The company continued to use them in their laboratories worldwide as far as 1998, thirteen years after the disk's introduction onto the market.
Resident Evil Zero
"The label reads "Security System ver. 1.02". It's probably used to activate some sort of security system."
In the Umbrella Executive Training School, an MO disk is needed to operate a terminal that gives a code to open certain doors of the facility.
Resident Evil /Remake
"It seems to be a start-up disk."
— Item description 1996.
"It appears to be a disk for rebooting a system of some sort."
— Item description 2002
In the Arklay Laboratory, access to the prison cell was denied without the planting of all three MO disks into a reader. Each was kept in level B3 - the morgue; fuel storage lab and power rooms. To get the best possible ending, all three disks were required. In the original game, the MO disks were needed to get each of the Pass Codes to use for the prison area, while in the remake, the MO disks were needed to directly disable the security protocols of the prison locks.
Resident Evil 2
"The label says "For Cargo Room Verification"."
Resident Evil Outbreak File #2
Two MO disks were recovered by outbreak survivors in the Umbrella R&D Center: the first was used to take up information and program into the Tyrant R before its activation, while the second contained information on people experimented on with the t-Virus.