The 1891-O Morgan dollar is a popular date, and it is plentiful enough in circulated and lower uncirculated grades. However, it is very tough in grades above MS64, with populations dropping sharply in MS65 or higher to where those pieces are considered quite rare. An extremely small number are known in MS66, and these draw huge premiums when they hit the market. PLS and DMPLS are rare, and those in the MS65 level or better are nearly esoteric.