Our chief references to the historical Owain appear in the poems of Taliesin, Urien's bard. In one poem, he appears as the victor of the Battle of Alclud Ford. Another, Gweith Argoed Llwyfain ("The Battle of Argoed Llwyfain"), tells of Owain's part in a battle between the men of Rheged under Urien and the men of Bernicia under "Fflamddwyn" (Firestealer), possibly the Anglian king Theodric. When Fflamddwyn demands hostages, Owain shouts defiance and inspires the men of Rheged to fight rather than give tribute to the English.
Taliesin also composed Marwnad Owain, an elegy to Owain. In the poem, it is said that Owain slew Fflamddwyn:
He inherited the kingdom of Rheged when his father was killed, but was immediately beset by his British neighbours, Gwallawc Marchawc Trin of Elmet and Dunaut Bwr; the former attacking Owein's brother Elffin whilst Owein and Pasgen, another brother, fought Dunaut.
Owein was slain in battle against his father's old nemesis Morcant Bulc of Bryneich after a short reign. His death marked the end of Rheged as an effective power.
He is said to be buried at Llan-Forfael or Llan-Heledd, which local traditions place in the churchyard of St. Andrew's at Penrith, though it is possible that the grave is actually that of the later Owen Cesarius.
Owein is said to be the father of St. Kentigern Garthwys by Denw, the daughter of "Leuddun", commonly identified with Lot of Lothian.
Over the centuries, the history of Owain known to storytellers faded sufficiently that he was incorporated into Welsh Arthurian legend and stories about him spread to continental Europe. Chrétien's Yvain, the Knight of the Lion and the related Mabinogion story Owain, or the Lady of the Fountain are devoted to his exploits, and he appears prominently in the Mabinogion tale The Dream of Rhonabwy and briefly in Geoffrey of Monmouth's Historia Regum Britanniae. The character is portrayed as an excellent knight in the later romances, the Lancelot-Grail cycle and Sir Thomas Malory's Le Morte d'Arthur, under one spelling of his name or another.
In The Dream of Rhonabwy, he plays a game of chess against Arthur while Rhonabwy looks on and the Saxons prepare to fight. The outlines of Chrétien's Yvain, the Knight of the Lion and Owain, or the Lady of the Fountain are essentially the same; Owain hears of a magical storm-making fountain in the forest of Brocéliande and seeks it out, only to find it defended by an excellent knight. He defeats this warrior and marries his wife Laudine, but forsaking his marital duties for knightly exploits, he loses her love. With the aid of a lion he rescues from a serpent, he completes several adventures and is eventually reunited with his lady. He appears in most of the later accounts, his importance indicated by his close friendship with Gawain and the passage in the Mort Artu section of the Lancelot-Grail cycle where he is one of the last knights to die before Arthur.
Almost all versions of the Arthurian story have Owain as Urien's son and Arthur's nephew, and the later accounts assume his mother is Morgan le Fay, if not one of the King's other half sisters. He has a half brother called Owain (or Yvain) the Bastard after him, the product of a union between Urien and his senechal's wife. The Welsh give him a twin sister, Morvydd, and as Arthur's maternal nephew he is a cousin to Gawain and the Orkney clan.