Meaning little deer, this Irish name is pronounced osh-een or ush-een. I love that it sounds sort of like ocean.
Oisín is known in Irish legends as the son of Finn mac Cumhaill and Sadbh, the greatest poet in Ireland and a great warrior. In his most famous tale he falls in love with a fairy woman, Niamh of the Golden Hair. They lived together for 300 years in the land of the young. When he decides to visit his old home, the 300 years catch up with him an he dies, never returning to Niamh.
James Macpherson published a series of poems in which Ossain (based off legendary Oisín) is the narrator. Osheen is a 3rd acceptable spelling, and probably the one that would cause the least amount of confusion.