[whpalmer4]

I have a rule of thumb I follow: if I need to do XYZ, and RobTrew has written a script that does XYZ (or something very similar), I install Rob's script and look no further.

The only problem with that plan here is that you may not have Hazel and Day One. It's possible to tinker with Rob's script a bit to remove the need for Day One and just dump everything in a text file (it actually already does that, but doesn't retain the file past the end of the day), and it isn't absolutely necessary to use Hazel to trigger the script. By far the path of least resistance would be to simply buy Hazel; it can be used for many other tasks besides this. Day One you may or may not want; I'm willing to provide a copy of Rob's script that simply appends endlessly to a text file rather than shoveling the data off to Day One.

Before you go down that path, however, you might consider how pressing your need is to have this information outside of OmniFocus. There's a built-in Completed perspective which will show you all the completed actions in your database. You can turn on a column that shows the date and time of completion via the View->Columns->Completion Date command. If you just select a bunch of actions, Edit->Copy, switch to your favorite text editor or Mail application, Edit->Paste will give you the text of the action names on separate lines.