The son of Finn mac Cumhail and Sabhdh, and father of Oscar, he was lead away to Tir na nOg by Niamh, daughter of Lir (some sources say Manannan mac Lir), where they spent three hundred years in peace and happiness (thus enabling Oisin to escape the destruction of the Fianna), until one day, when he became homesick. He climbed upon a magic horse, and rode across the waves to Ireland, where now he found all his people dead. Those who inhabited the island were small, and stood in awe at the sight of this giant warrior. He inquired to the fate of his family and friends, only to be told that they were long dead.
As he rode Ireland, looking for signs of his past, he saw an old man trapped under a rock. Naturally, he got off his horse to help the man. In doing so, as his foot touched the land, he suddenly became old and weak. Three hundred years suddenly rushed upon him, and left him lying on the road. But along came St. Patrick, who inquired as to who he was. "Oisin, son of Fionn," he said, and Patrick proceeded to ask him about the Fianna, and life in Tir na nOg, and Ireland under the famous Cormac mac Airt.
Oisin's tale is similar to those of Connla and Cormac mac Airt.
The story has been preserved in "The Colloquy of the Old Men," dating to about 1200 AD.
In the eighteenth century, James MacPherson made up a series of poems which he attributed to "Ossian," son of "Fingal"--however poetical they were, they were still forgeries. He claimed to be translating ancient Scottish Gaelic manuscripts, which never materialized once Samuel Johnson challenged him to show them. However, it's worth noting MacPherson wasn't entirely inventing these works, but drawing on Highland folklore, much as Iolo Morgannwg didn't so much invent but utilize Welsh folklore in his manuscripts. At any rate, the Ossian figures are as follows:
Ossian = Oisin
Fingal = Finn mac Cumhail
Dermid = Diarmuid Ó Duibhne
Ros-cranna = Grainne
Dar-Thula = Deirdre
Cuthulinn = Cuchulainn
Temora = Tara
And so on. But some of the stories are also MacPherson's invention.