A DOC is a batch of flash memory with a bios wedge to make it look like a DOS disk to the machine it's inserted into.
Under the DOS based versions of Windows (i.e. Win 95/98/Me) it should work okay- not too sure about the NT based
versions, but I think they work as well (Reading the OS support list at M-System's site indicates that I'm right,
but never having done it myself, I can't vouch for how easy/hard it might be...). Linux offers driver support
either through a loadable module provided by M-Systems or by way of the MTD driver layer in the 2.4.X and later kernels.
If you're using Windows or DOS, it's a simple matter of booting a hard-disk with your chosen OS and decanting
the OS you want on there by way of a file copy and putting a bootloader on the DOC (either via fdisk /mbr or other
means...)
If you're doing a Linux version, you'll need a patched version of the GRUB or LILO bootloader, an installed distribution
on a hard-disk with the M-Systems driver built for it or the Linux kernel having been compiled for Memory Technology Device
(MTD) support, and an idea of what you want to copy over as the distribution is very likely to exceed the capacity of the
DOC.
http://www.m-sys.com is a good place to start for how to actually go about doing this- use the 2000 series information
as that is what you're dealing with on a Websurfer PRO.