When you turn on the backup function in OmniOutliner's general preferences, a backup version of your outline is kept in the file as you work on it. In the case of a crash, power failure, or other nasty event, the backup version will remain in the file. When you open the file again, you'll be asked whether you'd like to use the last saved version or the automatically-saved backup version.

There are a couple of catches: If you have an outline open with unsaved changes, and you send the file to someone else, the file will contain your unsaved changes in the backup version; the other person will be asked whether to use the backup version. Please save your files before sending them around. A similar effect could happen when two people are accessing one file at the same time, over a network. In that case it's best to turn off the feature so as to avoid colliding with other people's changes.

If the backup function is turned on, an indicator may briefly appear in the center of your document to indicate that the file is being backed up.