[Lizard]

SETUP:
Step 1: Add a custom data column to your OmniPlan task inspector called "File"
Step 2: Copy the script below into a text file, and call it something like "Open File.scpt"
Step 3: Put that script file into ~/Library/Scripts/Applications/OmniPlan
Step 4: Open the "Customize Toolbar" button in OmniPlan, and drag the script up into your toolbar somewhere.
Step 5: Add file paths to the File field of your tasks.
NOTE: This script has no error handling in it. If you don't type a path exactly right, when you go to open a file, you'll either get a vague message that Finder had an error, or the script will just do nothing. The script expects paths like "/Applications/OmniOutliner.app" or "/Volumes/Tiger/Users/myName/Desktop/example.pdf".

SCRIPT:

tell application "OmniPlan"
	set taskList to selected tasks of front window
	repeat with t in taskList
		set customData to custom data of t
		set fileToOpen to (|File| of customData as POSIX file)
		tell application "Finder"
			open fileToOpen
		end tell
	end repeat
	
end tell

Now, when you want to actually open the file or folder associated with a task (or tasks), just select the task(s) and click the toolbar button.