[Andrew]

I can't think of any way to have OmniOutliner handle that automatically. The easiest workaround would probably be to do a global search and replace in your original TextEdit document (maybe copy/paste the pertinent portion to a temporary new document, to avoid messing up your original) to remove the leading dash+tab (or replace it with dash+space; OmniOutliner sees the tab as a column marker, but it will interpret leading dash+spaces as simply a starting bullet, and will remove them).