The outline architecture from OmniOutliner is used in a number of our apps: not just OmniOutliner, but also OmniPlan, OmniFocus, and now OmniOutliner for iPad.
For OmniOutliner 4 we took apart the entire architecture, rebuilding it on top of more modern OS X infrastructure (including CoreAnimation and bindings) and adding more flexibility (such as being able to have multiple views onto the same outline, with separate filters applied to each view). Of course, all of this rebuilding didn't just affect OmniOutliner 4: we had to get the outline architecture back together and working again before we could ship major updates to products which use the outline, such as OmniPlan 2 or OmniOutliner for iPad.
This week we reached a big milestone: the rebuilt outline architecture from OmniOutliner 4 is publicly available for the first time in our beta release of OmniPlan 2. Of course, that outline view is only part of the work necessary for OmniOutliner 4: we also have to read and write OmniOutliner documents, implement support for column summaries, sorting, etc. But much of that work was also necessary for building OmniOutliner for iPad, which is also very close to being ready.
So, yes, it's comingójust not before OmniPlan 2 or OmniOutliner for iPad. Fortunately, both of those should be shipping soon, which will let us focus more of our direct attention on OmniOutliner 4 itself.