Wow, pretty neat grease filled die. Not sure if it is really worth any premium, but would love to learn.
It is a very nice looking coin, and I am sure that whatever happened to the O happened in the mint, so, yes it is a mint error. Now as to whether it is a mint error that adds value, let's wait for some of the mavens to chime in.
Looking back at the coin, it kind of looks like the other O in EXPOSITION is starting to fill at the top...I should check mine.
I do not like this kind of an abnormal on a higher grade coin. There is a point where it can detract from it's desirability for some collectors.