Oisín, Osian, Ossian, or anglicized as Osheen was regarded in legend as the greatest poet of Ireland, a warrior of the Fianna in the Ossianic or Fenian Cycle of Irish mythology. He is the demigod son of Fionn mac Cumhaill and of Sadhbh, and is the narrator of much of the cycle and composition of the poems are attributed to him.

Fianna were small warrior-hunter bands in Gaelic Ireland during the Iron Age and early Middle Ages. A fian was made up of freeborn young males, often from the Gaelic nobility of Ireland, "who had left fosterage but had not yet inherited the property needed to settle down as full landowning members of the túath". For most of the year they lived in the wild, hunting, cattle raiding other Irish clans, training, and fighting as mercenaries. Scholars believe the fian was a rite of passage into manhood, and have linked fianna with similar young warrior bands in other early European cultures.