
Run OnyX.

In the OnyX menu, click "Preferences..."

In the Preferences window which appears, select the "Behavior" tab.

Check the "Force restart/shut down" option.
After some maintenance and cleaning operations it is mandatory to restart the computer. After running such tasks, OnyX displays a message asking what you want to do (Cancel, Restart or Shut down). You can also automatize startup and shut down from the "Behaviour" tab of the OnyX Preferences window.
If OnyX doesn't want to restart or shut down:
Run OnyX.
In the OnyX menu, click "Preferences..."
In the Preferences window which appears, select the "Behavior" tab.
Check the "Force restart/shut down" option.
Warning: The Force restart/shut down option restarts or shuts down the computer without displaying any alert message. If you have opened ad unsaved documents, your last changes will be lost for good.