Kind of looks like an O on top of the D. I would suspect someone opened up a new Mint in an area that has an O for a name. They don't have enough to make their own coins so they just stamp the O over someone elses.
Now there also is the possibility that the Denven Mint created a new format for coins that are not fit for distribution called DO OVER's. Then that would explain the DO appearance on that coin. It was stamped DO for Do Over.

Of course there is always the vague possibility that the other explanations are correct.