Saving an outline

While you are editing an outline, a diamond shape will appear in the title bar next to the document title. A solid diamond ◆ means that the document's content has changed since the last save, and that you will be prompted to save your work if you try to close it. A hollow diamond ◇ means that only the view of the document, such as the current scrolling location or the expansion of rows has changed; you can save this view information but you will not be prompted to do so when closing the document.
When saving an outline from the File menu's Save As... command, or the first time you save a new outline, you are presented with a sheet that lets you choose a file format for the outline.
OmniOutliner 3 is the default format, and preserves all of the structure and formatting of your outline.
OmniOutliner 3 Template is identical to the regular OmniOutliner 3 format, except that the file is meant as a starting point for new documents. When you open a template file, you are editing a copy of it while the original remains as it was.
OPML is a standardized outline format that will represent an outline's structure but not its formatting.