Is that Onyx telling you or something else?
Again, you shouldn't use Onyx to maintain your mac. OS X does NOT need maintenance since it has its own maintenance scripts that it runs automatically. Maintenance apps might do more harm than good and it might have bugs that might give you wrong information. Unlikely, but still possible.
Anyways, here is what you need to do:
Step 1: Ditch onyx.
Step 2: Open up Disk Utility and click on your Hard drive icon on the left and click "Verify Disk" or "Repair Disk". If OS X indeed reports that the disk needs to be repaired then:
Step 3: Backup.
Step 4: Then shut down the computer, hold CMD+R while booting up. In recovery mode, you should be able to use Disk Utility to repair the disk. If it's successful, then no need to worry anymore.