The Organisation Armée Secrète (OAS, "Secret Armed Organisation") was a far-right French dissident paramilitary and terrorist organisation during the Algerian War. The OAS carried out terrorist attacks, including bombings and assassinations, in an attempt to prevent Algeria's independence from French colonial rule. Its motto was L’Algérie est française et le restera ("Algeria is French and will remain so").

The OAS was formed from existing networks, calling themselves "counter-terrorists", "self-defence groups", or "resistance", which had carried out attacks on the Algerian National Liberation Front (FLN) and their perceived supporters since early in the war. It was officially formed in Francoist Spain, in Madrid in January 1961, as a response by some French politicians and French military officers to the 8 January 1961 referendum on self-determination concerning Algeria, which had been organised by President de Gaulle.

By acts of bombings and targeted assassinations in both metropolitan France and French Algerian territories, which are estimated to have resulted in 2,000 deaths between April 1961 and April 1962, the OAS attempted to prevent Algerian independence. This campaign culminated in a wave of attacks that followed the March 1962 Évian Accords, which granted independence to Algeria and marked the beginning of the exodus of the pieds-noirs, and in Jean Bastien-Thiry's 1962 assassination attempt against president de Gaulle in the Paris suburb of Le Petit-Clamart. Another prominent target was the existentialist philosopher Jean-Paul Sartre, who supported the FLN.

The OAS still has admirers in French nationalist movements. In July 2006, some OAS sympathisers attempted to relight the flame of the Tomb of the Unknown Soldier to commemorate the Oran massacre on 5 July 1962.