During the joint German-Soviet invasion of Poland, which started World War II in September 1939, the town was invaded by Germany. Already during the invasion, the Germans committed the first executions of local Poles (see also "Nazi crimes against the Polish nation"). Under German occupation, the town was annexed directly into Germany. Poles from Olkusz were among the victims of a massacre committed in Celiny in June 1940. On July 16, 1940, in retaliation for the killing of a German policeman, the Germans carried out a massacre of 20 Polish hostages. On July 31, 1940, the Germans gathered all Polish and Jewish men aged 15 to 55 and subjected them to long tortures and humiliation. These events are known as the . The German terror against the local population was aimed at the planned Germanisation of the town and the region. The occupation ended in January 1945, and the town was restored to Poland.