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.

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 150000Taler.

[1] On 14 April 1621 in Wolfenbüttel, Otto III married Hedwig (1580-1657), the daughter of Duke Julius of Brunswick-Wolfenbüttel.

This article about a member of the German nobility is a stub.