General Preferences
When OmniOutliner Starts Up - The standard behaviour for applications on Mac OS X is to create an empty document on startup. You can turn this off for OmniOutliner with the Create a new document checkbox. You can also have OmniOutliner automatically re-open any documents you had open the last time you quit it.

New Document Settings - You can change several default options that will be used when creating a new document;
Related Topics
Table of Contents: Using OmniOutliner
OmniOutliner Features
OmniOutliner's Info Panel
OmniOutliner Menus
OmniOutliner Preferences
Keyboard Preferences
Text Export Preferences
RTF Export Preferences
Update Preferences