Pretty clear to me:
Stealth = defragging without making your system unusable during the defrag.
Space = not really a defrag, just consolidation of free space
Complete variations = normal defrag with files arranged based on name or date modified
Complete/access = defrag with files arranged based on how recently they were accessed
Complete/access is essentially what a normal Windows defrag does, since Windows tracks file usage and arranges files for fastest access based on usage patterns. I assume that O&O will use the logging function of Windows to arrange the files according to that access, rather than just putting the most recently accessed files at the front of the drive (Windows knows that often-accessed files should go ahead of a file that's accessed once but recently).
The complete/date variation would be good if you needed very quick access to certain files that are often modified (but which don't tend to get any bigger). They'd go at the front of the disk for faster reading and writing, but other files that don't change, like program files, might not be arranged optimally to reduce load times. If the important files get any bigger as well, parts of them will end up getting written farther out on the disk, possibly even all the way at the end, so performance gets reduced.