Otto was the eldest son of Albert II, Duke of Brunswick-Lüneburg. Otto and his brothers succeeded on their father's death in 1318; he served as his brothers' guardian while they were not of age. In 1323, he acquired the Altmark from Brandenburg as an inheritance of his wife's; but he sold it in 1343, when he failed to establish control there.
Otto died in 1344 in Göttingen and was succeeded by his brothers.
Firstly, Otto married Jutta (died 1317), daughter of Henry I, Landgrave of Hesse. Secondly, he married Agnes (1297–1334), daughter of Herman, Margrave of Brandenburg-Salzwedel, in 1319. From the first marriage, he had one daughter: Agnes (died 1371).