Ron Guth: The 1899-O Micro O has a very tiny mintmark on the reverse that is much smaller than the mintmarks normally used on Silver Dollars. Breen wondered whether the Micro O was a mintmark from a Quarter Dollar. Micro O's comprise three different VAM varieties: VAM 4, 5, and 6. The 1899-O Micro O is a legitimate variety and should not be confused with the counterfeit Micro O's of 1896, 1900 and 1902.
The 1899-O Micro O is not nearly as rare as it once was believed to be, but because of the tremendous collector demand for Morgan Dollars, interesting varieties such as this are snapped up. As of September 2011, PCGS had certified over 600 1899-O Micro O Dollars, but only 29 were Mint State. The finest example certified by PCGS is a single MS-65+.