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Tell you what for a small donation to my kid's college fund, I'll make sure that the next version of OmniFocus does this. I feel confident that I can deliver on this promise, even though I don't work for Omni, because it already does this :-)

Select an action and look in the Inspector. Or turn on the Completed date column in the outline via the little gear menu at the upper right. You can even make a perspective that shows this column for use when looking at completed actions, and not have it displayed the rest of the time.