What is the history of a 1921-O Morgan dollar?

I am not aware of a 1921 Morgan Dollar that carries a mint mark of "O." However, the 1921 series do carry mint marks of "D'" and "S." However, what is interseting about the 1921 Morgan Dollars is that they were produced by the US mint after a hiatus of 17 years - secondary to the The Pittman Act that required that all silver dollars be melted. More info -

There could not be a 1921-O dollar because the New Orleans mint closed in 1909.

1921 was the only year that Morgan dollars were struck in Denver, because the Denver mint opened in 1906, two years into that hiatus.