Otto was a son of Duke
Otto II of Brunswick-Harburg (1528-1603) from his second marriage, with Hedwig (1535-1616), the daughter of Count
Enno II of East Frisia. After the death of his brother
Christopher in 1606, Otto III and his brother
William Augustus ruled Harburg jointly. Their joint reign was described as harmonious. In a treaty of 11 January 1630, the brothers renounced their right to succeed in
Brunswick-Lüneburg, in favour of
Christian, in exchange for Christian paying their debts, which exceeded . On 14 April 1621 in
Wolfenbüttel, Otto III married Hedwig (1580-1657), the daughter of Duke
Julius of Brunswick-Wolfenbüttel. This marriage remained childless. == References ==