Can't import from OmniOutliner

I am using OF 2.12.4 and OO 5.3.4; Mac OS HIgh Sierra. When I try to impiort an outline, and the browsing list opens, all the OO files are greyed out, so I can’t import them. Any ideas?

I don’t think you can get OF to import the files, but what I think you can do is open both apps and drag drop an outline from OO to OF.