[aseidlitz]

Got it working: this version removes the WBS numbers e.g. if you need to renumber the tasks or after exporting to WBS.

tell application "OmniPlan"
	tell front document
		repeat with MyTask in tasks
			tell MyTask
				set tid to AppleScript's text item delimiters
				set AppleScript's text item delimiters to ") "
				set t to name as string
				if (count t's text items) > 1 then
					set t to t's text items
					set t to items 2 thru -1 of t
					set name to t as string
				end if
				set AppleScript's text item delimiters to tid
			end tell
		end repeat
	end tell
end tell