[DerekM]

The behavior of backups in OmniOutliner depends on what OS you're using. The help files describe the pre-10.7 behavior. On 10.7 and 10.8, we use Apple's Versions ability to autosave for you and store past versions of the document. But, has whpalmer4 suggests, if you want actual backup copies of your files, you will need to use Time Machine or some other backup system.