Ossian or Oisín was a Gaelic bard. He supposedly lived during the 3rd century, and was born at Loch Achtriochtan. He was the narrator of the Celtic legends of Fionn mac Cumhail, also known as the Fenian Cycle or the Ossianic Cycle, one of the four great cycles of Celtic mythology.

First of all, the stories are legendary, and similar to the stories of King Arthur, it is not sure if there is a real person and story behind the legend or not. Second, the legendary bards were, similar to the Medieval Troubadours or Minnesänger, all together: poets, singer, and warrior. And to make it completely mind-boggling, there are numerous versions of the legends, including one where Ossian and Fionn meet for the first time, when Ossian is already an adult. The whole legend is written with Ossian as the narrater and so it is considered that he is the author.

Oisín means little deer, because his mother Sadbh has been turned into a deer by the druid Fer Doirich. Fionn was hunting and caught her. As he did not kill her she was released and became human again. Fionn gave up hunting and fighting, and lived with Sadbh, and she was soon pregnant. But Fer Doirich again transformed her into a deer and she returned into the forest. Tradition says, her son Ossian was born near Loch Achtriochtan in a cave which is now called Ossian's Cave or cave of Ossian. Seven years later Finn found their child naked on Ben Bulben.