Let me first say thank you for making it possible to export to OmniOutliner. It would be equally convenient for someone like me to be able to import from OO as well. I tend to use OO for quick note-taking – its interface is simply the easiest thing I have ever found for such purposes. But as a project expands and as I begin to do more research, I find OO’s interface is also its limitation. That’s where I am now scaling up to DT Pro.
Perhaps there will be a script or Automator action to import text from OO 3 in the future.
In the meantime, here’s a general workaround for both linking to and capturing the text of unsupported file types:
 In DT/DT Pro, create a new rich text document (Command-Control-N).
 From the Finder, drag a file name or icon onto the text window of the new document. A double-clickable link to the external file will be created.
 While viewing the file in its native application (OmniOutliner 3.x, for example), select and copy to the clipboard text and images and paste the clipboard contents into the DT/DT Pro document you have created for this purpose. Or, select material in the OO 3.x document and drag it onto the corresponding DT/DT Pro document window.
Now you have in DT/DT Pro a link to the unsupported document type, and its text content can be searched and analyzed in your database.
In addition to Bill’s very useful suggestion above, it’s also possible to export from Omni Outliner 3 in several file formats that can be directly imported into DT(Pro).
OO can export an outline as RTF. This will preserve all of OO’s content, but will of course result in a “flat” file that has lost its outline characteristics (parent/child relationships, dragable hierachies, etc.)
OO can also export a file as OPML. OPML can be directly imported into DT Pro, with a unique result: your outline document is converted to a group, with each item in the hierachy converted to a document, or a sub-group. This result conforms to the way that DT (currently) creates and manages outlines made from with DT. If you have any text comments in an OO column, you’ll probably lose them. However, your outline wil be preserved, albeit in a folder/file format.
I use also another tool for notetaking and writing. I m not “importing” the notes, text etc. to DTp. I use the OSX services. Fast and easy. Works perfect for me.
Select all the text (Services Deventechnologie has a small think to make your one Service menu in Cocoa Apps, what OO is)
Take plain/rich note. Voila its in DT.
There is a short cut to.
(Or with sservices)
Append plain/rich note
I send in a few minuts half a book to Dt in that way. The text modules organised in file(s) like I need them.
If you are once trained to that, you will forget about export and import
hope that helps