If you have devices that you would prefer to remain powered after OMV shuts down on a loss of mains, finding the necessary configuration requires a little digging and use of the sdorder directive, which isn't covered in most of the basic tutorials on NUT.
Here is my Driver configuration directives using my APC SmartUPS:
The drawback to this configuration is that if the power returns before the UPS shuts down on low battery, then the OMV server will not be turned back on since it will not detect any change to the AC state (assuming that the OMV server is configured as such).