Still a minor when he succeeded to the throne, he initially had to accept the tutelage of his cousin Frederick I, ruling Prince of Brunswick-Wolfenbüttel, until he was declared to have reached majority by King Wenceslaus in 1398.
For the maintenance of law and order, he allied with the cities in the region, such as Uslar, Seesen and Gandersheim, to fight against powerful robber barons.
He succeeded in 1407, together with the citizens of Göttingen 1407, in storming the castle at Jühnde, while also forcing the Lords of Adelebsen, Hardenberg and Schwicheldt to respect the public peace.
When Duke Frederick I of Brunswick-Wolfenbüttel died in 1400, Otto had to sign an inheritance treaty with his surviving brothers Bernard I and Henry the Mild.
After lengthy negotiations and pressure exerted by King Albert II of Germany, the dispute was finally settled in 1442: Otto retained the city and castle of Uslar, his wife received Münden, Dransfeld and Sichelstein.