The Special Organisation (French: Organisation spéciale) was a secret paramilitary organisation in colonial Algeria, founded by Mohamed Belouizdad of the Movement for the Triumph of Democratic Liberties (MTLD) in 1947 to prepare for armed struggle against France, which ruled Algeria as a colony since 1830. The turn towards guerrilla warfare was in large part encouraged by the reactions to the Sétif massacre in 1945, and other examples of violent repression, which had convinced many Algerian activists that peaceful political work would be pointless.
The OS had around 1500-2000 members at its peak, and spawned the groups that would later form the FLN; this group, in turn, became the leading force in the Algerian War of Independence (1954–1962), and later Algeria's single ruling party until 1989.
The OS was dismantled by French police in 1951, and many members imprisoned. Only the units in Aurès and Kabylie remained active at the time.