[1][2] Otto was born c. 1305[3][4][note 3] as the eldest son of Count Henry I of Nassau-Siegen and Lady Adelaide of Heinsberg and Blankenberg [nl].

[1] A new division treaty followed on 18 June 1341,[5] which assigned to Otto the Siegerland, the Mark Herborn with Dillenburg and the district of Haiger, as well as Löhnberg.

[6] On 20 September of that same year, Otto was granted city privileges for Dillenburg by Holy Roman Emperor Louis the Bavarian.

To control his expenses, he was forced to pledge possessions frequently and as a result the development of a powerful activity inwardly as well as outwardly was hampered.

[1] In his last feud, against the brothers Gottfried and Wilderich III von Walderdorff [de], Otto was killed[2][8][9] in a battle, that, according to charters, must have taken place between 6 December 1350 and 25 January 1351.