[Lizard]

You can customize the toolbar -- it's in the View menu. The available toolbar buttons don't include indent/outdent or undo, but it does have several other "common functions".
Indent and Outdent are both available in the context menu on tasks. (Right-click the task, or select it and click the 'gear' menu at the bottom left of the window.)
You can also create your own custom toolbar buttons by writing AppleScript, and put them in Library/Scripts/Applications/OmniPlan.Then they will be available in the "Customize Toolbar" menu as well.