It is an "O" (not a 0) and it is the mintmark, found on the reverse of the coin under the eagle. It means the coin was struck at the New Orleans mint (the mint is no longer in operation today).
The San Francisco Mint
The O is the mint mark, it stands for the New Orleans mint.
SAN FRANCISCO
It's the mint mark for San Francisco.
Where it was minted. D=Denver
It means that the coin was produced at the Denver Mint.
The BU stands for Brilliant Uncirculated for all coins not just Morgans dollars.
It stands for James Earle Fraser, the designer of the Buffalo nickel. Since the buffalo silver dollar borrows the design of the buffalo nickel, Fraser's initial is placed on it also.
it stand for John Pierpont his full name was John Pierpont Morgan
John Pierpoint Morgan
John Pierpont Morgan
The intertwined "A F" under Miss Liberty's neck is the monogram of the designer Anthony de Francisci