O&O Defrag can be instructed to always start when the screen saver is active. A defragmentation will be run automatically, if your computer is not in use.
Create a new job for doing this and in the Schedule tab, select Frequency/ScreenSaver. All other settings are specified by O&O Defrag itself.
As soon as the screen saver ends (login, etc), the running defragmentation stops.
Note: Make sure that the screen saver is enabled. Otherwise, it is not possible to start the O&O Defrag screen saver mode. More information on enabling the screen saver can be found in the Windows help file.