Otto was the eldest son of Henry II, Duke of Brunswick-Grubenhagen (c. 1289 – 1351) and Jutta of Brandenburg.
On 25 September 1376, Otto married his second wife, the three times widowed Queen Joanna I of Naples.
The so-called Western Schism started in 1378 with the election of two rival popes, Urban VI of Rome and Clement VII of Avignon.
With no hope of having further children of her own, Joanna chose to strengthen her alliance with France by adopting Louis of Anjou, younger brother of Charles V, as her heir.
On the 27th of July, 1382 at age 56, Joanna was strangled in prison in the Castle of Muro Lucano held by Palamede Bozzuto.