Øverland was a communist from the early 1920s, but changed his stand in 1937 partly as a reaction to the Moscow Trials. He was an avid opponet of nazism and in 1936 he wrote the poem "Du må ikke sove" ("Dare not to sleep!") printed in the journal Samtiden. It ends with "Jeg tenkte: Nu er det noget som hender. Vår tid er forbi - Europa brenner" ("I weighed [thought]: Something is imminent - and it’s dire. Our era is over — Europe’s on fire!"). The probably most famous line of the poem is "Du må ikke tåle så inderlig vel den urett som ikke rammer deg selv!" ("You cannot permit it! You dare not, at all. Accepting that outrage on all else [other people] may fall!")
During the German occupation of Norway from 1940 in World War II, he wrote a series of poems which were clandestinely distributed, leading to the arrest of him and his wife Margrete Aamot Øverland. Arnulf Øverland was held first in the prison camp of Grini before being transferred to Sachsenhausen concentration camp in Germany. The poems were later collected in Vi overlever alt ("We survive everything") (1945).
After the war, Øverland became a noted supporter for the conservative written form of Norwegian called Riksmål, he was president of Riksmålsforbundet (an organization in support of Riksmål) from 1947 to 1956, playing an important role in the Norwegian language struggle in the post-war era.
In addition, Øverland adhered to the traditionalist style of writing, criticising modernist poetry on several occasions.