The way to do this is to use cut and paste.
In OmniOutliner, select the items you want to move to OmniFocus and copy them.
In OmniFocus, make sure you're in Planning mode. Select a project if you want the top level in your outline to be projects, or the action in your project after which you want the pasted items to appear if you want them to be actions. Now paste.
You should get a dialog asking how to map the columns in OmniOutliner to the fields in OmniFocus. The Topic should map onto Title; if you have other columns in OmniOutliner, you can use them for context, start date, due date, duration, and metadata (which is the Notes field).
After setting your column mapping, press OK, and the stuff you copied from OmniOutliner should be in OmniFocus as actions or action groups. (Or projects, if you pasted at the project level.)