His brother Eric predeceased both Otto and his father, fighting to rid Denmark from the rule of the counts of Holstein.
King Christopher died in 1332, leaving Otto and his younger brother Valdemar as his only sons and potential heirs to the throne.
In 1332, Otto made an attempt to win the Danish crown, but was defeated by Count Gerhard III of Holstein on Taphed at Viborg on 7 October 1334.
Upon his father's death and his failed attempt to seize the throne, Denmark ceased being a formal kingdom, and for the next eight years it was subjected by various mortgagees to German military rule.
Once Valdemar became king in 1340, Otto was released on the condition that he surrender his claim to the throne.