What You Can Do with It
When it first shows up, OnyX is a toolbar that requires the system's password to activate it.
Clicking Appearance brings up a dialog box with tabs that give you control over the appearance of core programs. Instead of having to munge around each of the programs, most of what you might want to specify is aggregated into one place.
Maintenance, cleaning, and automation have overlapping functions but relate to using the file cleaning and rotation utilities built into Unix. Other programs have addressed using these functions before (notably MacJanitor and Cocktail), but OnyX gives you different ways to work with them in its dialog boxes. You might want to experiment with different workflows to see what works for you.