I love Oisín and always have. It's a super lovely, world class name, and the deer imagery does not hurt one bit.
Osheen looks more like a contraction of a last name like O'Sheen, and
Ossian seems like it would be pronounced OSH-un. I wouldn't want to bother having a name spelled as unintuitively (in the United States) as Oisín, though--if the US is where you are.