There have been threads about special operating systems for the SAFE network. Those are about local operating systems it seems. A distributed OS on the other hand would run in the farmers and allow clients to use the entire SAFE network as a gigantic distributed operating system.
The advantage of a distributed SAFE OS is that it can replace centralized servers for big applications. Imagine a search engine for example. One could hardly put the whole Google Search infrastructure into a single SAFE network app. With a distributed SAFE OS, these kinds of huge systems will be much easier to develop.