[moarnao]

I ran into this last night actually. The solution is to open your default template instead of making a new document. Since OO recognizes that it's a template, opening it has the effect of creating new document.

-- Replace "Default.oo3template:" with your favored template.  Note the trailing colon, because the file is a bundle.
set template to alias ((path to application support from user domain as string) & "OmniOutliner 3:Templates:" & "Default.oo3template:")

tell application "OmniOutliner Professional"
   open template -- as opposed to "make new document"
   set newOutline to front document -- "open" doesn't return a reference to the new document like "make" does, so use this instead.
end tell