In the 1930s, Øisang was a key figure in developing the political platform for the Labour Party, having taken a reformist stance in the 1920s. He also continued in local politics, chairing the city school board from 1938. Øisang stepped down as editor of "Arbeider-Avisa" in 1940 as he in 1939 had been elected as party secretary of the Labour Party; his successor was Harald Langhelle. However, things were hampered by the Nazi German invasion and subsequent occupation of Norway. First, Øisang represented the Labour Party in the negotiations about administration of Norway after the invasion, as the cabinet Nygaardsvold had fled the country. However, the negotiations were not fruitful, and in September 1940 all political parties except for Nasjonal Samling were banned. The newspaper "Arbeider-Avisa" was shut down in 1941, and Harald Langhelle was executed following the martial law in Trondheim in 1942. Øisang, who had spent the occupation period writing books on local history, was arrested on 9 October 1944, was incarcerated at Vollan and then sent to Berg concentration camp a few weeks later. He was released in April 1945, one month before the liberation of Norway.