Óin (Third Age 2774 - 2994, 223 years old) was the son of Gróin and older brother of Glóin, one of the Dwarves of the following of Thorin II Oakenshield who reclaimed Erebor from Smaug.
He later travelled to Moria with Balin, and was lost in the ill-fated attempt to reoccupy Khazad-dûm. Óin went to seek the upper armouries of the Third Deep and also went to the Hollin gate, where he was taken by the Watcher in the Water.
Óin, may have also fought in the War of the Dwarves and Orcs, considering that his younger brother Glóin was a veteran of the war[source?].