DDO stands for:
- David Dunlap Observatory Catalogue, a catalog of the David Dunlap Observatory of the dwarf galaxies
- Deputy Director for Operations, former name of the head of covert operations within the Central Intelligence Agency (now the National Clandestine Service Director of the )
- German disc jockey organization, in 1963 founded company
- German Druid Order, 1872, was established club
- Dollard -des- Ormeaux, Canadian city in Quebec
- Dungeons and Dragons Online, an online computer game
- Ddo. , de date, from the date.