add completed date after the fact
 

I often make a task without adding a date or a context, but I want to record when the task is completed. Is there a way to add a "task completed" date?

There is a completion date/time for each action (and project), set by default to be the time when you check the completion box, and directly visible in the inspector. You can also edit the completion date/time from the inspector if you want to correct it.

The inspector is the only direct way I can think of to see the completion date/time in OmniFocus, but some of the sorting/grouping options will also give you a rough indication, either on a relative basis, or chunked (this was completed in the last week, that was completed yesterday).