The Omni I/O has 'Insert' points, which are after the preamp, but before the signal goes out of the Omni to your soundcard (Delta 66).
Look at the manual for the Omni to see how it's hooked up, but bacically, you plug the mic into which ever channel/preamp you choose on the Omni, put the RNC into the 'insert' point for that channel, and you're all set to compress/limit the mic signal before it goes into your soundcard/recording application.