Olyroun appears in the late 14th-century Middle English romance Sir Launfal, composed in the West Midlands dialect. In the tale, Launfal discovers that Tryamour, the lady he meets in a forest pavilion, is the daughter of the King of Olyroun and of Fayrye. Her father is described as the king of all the fairies of the Occident, the west, both far and near, a man of great might whose realm exists apart from the mortal world. Olyroun himself never appears directly, but his power is felt through every gift his daughter bestows. His island is a place of unimaginable wealth, entered through forests near Caerleon where two maidens lead Launfal to their mistress, who lies upon a bed covered with purple silk beyond the mortal world.

The romance tells how Sir Launfal, once the steward of King Arthur's court, fell from grace through the malice of Queen Guinevere. When Guinevere showed ill will to Launfal at her wedding feast, he left Camelot in humiliation and returned to Caerleon, where he soon spent all his money and sank into poverty. One Trinity Sunday, while he rested in a forest, two maidens led him to Tryamour. She offered him her love and magical gifts from her father's treasury: a purse of silk that never empties, an invisible servant named Gyfre, a horse called Blaunchard, and a banner bearing her arms. She promised that no knight's blow could wound him in war or tournament. All these blessings came from Olyroun, and Launfal accepted them on the condition that he keep their love secret.

The crisis comes when Launfal breaks his promise. Goaded by Queen Guinevere, who first tried to seduce him and then sneered that he preferred young men, Launfal declared that his mistress's ugliest handmaiden was more beautiful than the queen. At that instant, all the fairy gifts vanished. Guinevere accused Launfal of insulting her, and Arthur put him on trial. The court gave him a year and a fortnight to produce his lady as proof. On the day of judgement, Tryamour appeared at Arthur's court with twenty-four maidens, all more beautiful than any mortal woman. She proved Launfal's innocence, then breathed upon Guinevere and blinded her. Gyfre appeared with Blaunchard, and Launfal rode away with his beloved to Olyroun, where her father ruled in splendour beyond mortal kings.

The story of Olyroun is older than Chestre's poem. It comes from a 12th-century lay written in Anglo-Norman French, where the fairy mistress remains unnamed and the island to which the lovers escape is Avalon itself, the same mysterious realm where King Arthur would one day be taken to heal his mortal wounds. In that older version, the fairy king is not named, but his daughter's wealth and power are equally vast. The transformation of Avalon into Olyroun shows how Breton and Welsh fairy traditions blended with continental romance, as the island of Oléron near Brittany became a location for a fairy kingdom that had once been the Celtic Otherworld, where mortal time does not apply.

Olyroun endures as one of the unnamed powers behind Arthurian romance, the fairy king whose daughter's love redeemed a fallen knight and whose island represents the eternal temptation of the Otherworld. Launfal's annual return to joust with mortal challengers suggests that the boundary between the fairy realm and the human world is thin on certain days, and that Olyroun's kingdom is not entirely closed to brave men. Some say Olyroun represents a memory of Celtic sea-gods or Breton fairy kings, while others believe the name simply reflects the real island of Oléron, where storytellers placed their tales. Either way, he remains the sovereign of one of the most tantalising destinations in Arthurian legend, a king who needs no sword because his realm offers what no mortal kingdom can: eternal youth, endless wealth, and love that outlasts death.