Óin was King of Durin's line, successor to Glóin. In his time, the Dwarves of Durin's Folk still dwelt in the Grey Mountains of the north. In the later years of his reign, the Shadow returned to Dol Guldur in Mirkwood, and Orcs and other evil things began to multiply in the northern mountains. In Óin's last years, the Orcs plundered Moria, which was then deserted, and took it to dwell in. This was but the first of the calamities that were to befall the House of Durin over the coming centuries.
Óin was 250 years old when he died, having reigned as King for 103 of those years. He was succeeded by his son Náin, who became KingNáin II.