The House of Brunswick-Lüneburg did not recognize Otto's right to inherit his father's lordship on the grounds that the marriage between his parents had been morganatic.
Supported by Emperor Ferdinand I, Otto repeatedly renewed his demands and in 1560, he was finally confirmed as his father's successor as ruler of the Lordship of Harburg.
Otto continued his father's construction project at Harburg Castle and transformed it into a princely residence.
Otto did not allow Jews and Christians of other denominations to settle in his country.
On 8 September 1551, Otto married his first wife, Margaret (1530–1559), daughter of Count John Henry of Schwarzburg-Leutenberg.