The 1886 Morgan silver dollar keeps its value because of the relative rarity of attractive specimens that resemble uncirculated coins. The 1886-O came from the New Orleans Mint, and even though the run produced 10.7 million dollars, millions of those had sloppy strikes.
The morale at the New Orleans Mint in the late 19th century was low. That mint received less favor from Washington than the other mints, and recent years had seen coins simply put into bags and stored rather than put into general circulation. This low motivation caused sloppy die spacing.
Thanks to the 1918 Pittman Act, millions of silver coins were melted. Even so, many fewer 1886-Os have been released in subsequent Treasury actions than what would have been expected. In 1965, the value was $375 per roll, or $18.75 per coin.
In MS-65 grade, the 1886-O Morgan silver dollar is rare. Between 5,000 and 10,000 examples of MS-60 through 62 (reflecting slightly lower quality) exist, but at MS-64, perhaps 125 to 250 still exist. At MS-65, the 1886-O is the rarest Morgan dollar from New Orleans. One is known, and there may be as many as three. A common forgery has an "O" mark on the 1886-P (Philadelphia, minted with no letter), but those show a seam where the added "O" touches the field.