His daughter Fíriel married Prince Arvedui of Arthedain.
Ondoher and his eldest son Altamir led the main army close to the Black Gate where they clashed with the Wainriders.
The left wing of Ondoher's army was commanded by Adrahil, the Prince of Dol Amroth, the right wing was commanded by Minothar, the captain of Gondor's armies. The center was commanded by Ondoher himself.
Although Eärnil finally defeated the Wainriders it was learned that Ondoher's second son Faramir (not to be confused with Faramir, brother of Boromir) was slain with his father while law required that at least one heir was left behind.
This caused a constitutional crisis in which Arvedui and Eärnil claimed the throne. Although Arvedui had a stronger claim, the throne was granted by Steward Pelendur to Eärnil who would reign as King Eärnil II.