Otto was the eldest son of Landgrave Maurice of Hesse-Kassel (1572-1632)[1] from his marriage to Agnes (1578-1602), the daughter of Count John George of Solms-Laubach (1546–1600) and his wife, Margaret of Schönburg-Glauchau (1554–1606).

Poet Hermann Kirchner, later professor of rhetoric at the University of Marburg, wrote a poem to mark Otto's birthday.

[2] The Ottoneum, built in Kassel by Maurice in 1603, was the first theater building constructed in Germany.

He was in England from 23 June to 7 August 1611, with a retinue of about thirty, and was among the potential suitors of Princess Elizabeth.

John Milton, the poet's father, composed a song in four parts, for which the landgrave presented him with a gold medal.