[Andrew]

Unfortunately, there is no support for this in OmniOutliner 3. For what it's worth, we are aware that many people have requested this capability, and indeed we at Omni would love to have this as well. Also, some architectural work has gone into this (see OmniFocus, which shares some of that codebase and which has a limited form of this feature), so chances are good that we will be able to introduce it in a future version of OmniOutliner.

-andrew