Ozma is a super boss, much like weapons in other FF games. He's 100% optional. I find it funny how less ppl seem to be aware of him than the weapons of other FF games
He's in the chocobo sky garden or whatever it's called, and can't remember what you have to do to fight him. You should be able to fight him as soon as you have access there (that is, when you have a golden chocobo). But let me just say that he's incredibly tough, and a lot about whether you'll win is luck regarding what sequence of attacks he'll use.