Nisus Writer Pro. Nisus started perhaps before OmniGroup, and of the old classic Mac word processors, they appear to have been the only one besides Word that survives as a Mac app (Maybe Mariner write?)
Nisus is fantastic for my legal work. Uses RTF and PDF. Has GREP searching capabilities. Does things that even Word cannot do (specifically, it manages tables of authorities better than Word does). Handles links in PDF’s better than word, though not perfect. As far as Mac word processors go, there’s Word, Nisus and Mellel in the “heavy hitter” category, then everyone else. Each of those three does all the basics, and each does some things better than the other (Mellel, for instance, does multi-language and right to left text better than anyone).
In fact, Omni and Nisus Group have a small connection: LinkBack technology was invented by Nisus, and Omni scooped it up. Not sure who maintains it now, but LinkBack is really System 7’s Publish and Subscribe, brought to the modern age. It’s no wonder that Nisus, a very long standing Mac company, would want to bring that feature back to the future.