If you have the recovered files in the same directory, you can run this from Terminal.app
Code:
zgrep -l OUTLINE *.gz
And then this will list out any file that match. This should greatly narrow down the list of files to check by hand. Renaming the file from .gz to .oo3 should then allow you to open it in OmniOutliner. The one case where this won't work however is if you had attachments embedded in the file. In those cases, you'll need to rename the file to "contents.xml" and put it inside a .oo3 wrapper. Creating a folder, sticking the file inside, and renaming the folder to "foo.oo3" should work.