DDO - Doubled Die Obverse
DDO is shorthand notation signifying a Doubled Die Obverse (DDR indicates Doubled Die Reverse).
A doubled die may be created many ways. One cause of doubling is the process that transferred the image from a master die to a working die, hubbing, could introduce slight shifts forming doubling. Additionally, the date and mint mark were hand punched into Morgan dollars, allowing for shifts between the blows of the engraver’s puch.
Another type of doubling called strike doubling, machine doubling or mechanical damage doubling is actually caused in the striking process itself. This doubling has a flat, shelf like appearance and is not considered collectable like the “true” doubling from hub doubling or repunching.
A good discussion on mechanical damage doubling can be found here:
The most famous DDO in the Morgan series is the 1888-O VAM-4 "Hot Lips" variety.
Please click this link for the full VAMworld.com definitions page.