I have used OnyX a couple of times. Some of the tools you can use without much caution. To avoid causing a problem I would advice to use the help feature, to read about what the various checkboxes and buttons do.
I havn't used all the bells and whistles of OnyX, but as I understand, some of its features are safe to use, others can cause a problem, where there is none.
So pop open Help, from the Help-menu, before venturing too deep, and you will be fine.