Ozma is an optional boss that you can fight once you have a Gold Chocobo and go to Chocobo's Air Garden. Hardest boss in the game.

Ancient Aroma teaches the Odin's Sword ability. It's found in the basement of Ipsen's Castle, as Ouch! said. In order for it to work, I think you merely need to have the ability equipped when you summon Odin.