I would mostly agree with Jan_H.
The rich text file format (RTF) is suitable for documents that are text-based and that do not have demanding formatting or layout requirements. RTF files can be opened in just about any text editor or word processor. Of course, if you are using OmniOutliner, then your requirements are very likely beyond what can be done with a rich text document.
If you are creating a simple static outline, OmniOutliner may be overkill. OmniOutliner’s strength is in the fact that it has long surpassed being a on trick pony. I do not know how OmniOutlier was prior to v3, but it would not surprise me if v1 was not much more than a simple outliner. Today, OmniOutliner is powerful structural document generator that includes true parent-child relationships upon which its feature set is built; rich text applications do not have this capability.
The same is true of Microsoft Word in that it is overkill for composing simple text documents. Most people perhaps rarely use TextEdit (macOS) or Wordpad (Windows), but those simple word processors are excellent tools for creating simple text documentation.
Proprietary file types have a tendency to dramatically change over time, so a simple rule of thumb for any software is to keep your files up-to-date. Of course, that type of task can be quite time consuming and few applications have a batch conversation process.
The real irony for OmniOutliner users is the fact that OmniOutliner is the only outlining software that supports OPML; think of it as the RTF equivalent for hierarchal documents. So, we can save a copy of our .ooutline documents as an OPML file, but to date, OmniOutliner is the only outliner software that can open OPML files. The entire purpose of the OPML format is to permit transporting outlining documents between applications, but outside of its use on the Web—if you look up OPML you will see more about RSS feeds than anything associated with outlining software—it is slim pickings. That is very unfortunate.