Let me anwser that for you.
To get omnislash you need to go to the battle arena at the gold saucer and keep getting points until you can get omnislash. Knight of the round can be found at an islands in one of the top corners in the world map. You can only get on the island with a gold chcobo.
Lets live together, I don't know how long it will last, but it doesn't have to be for show, together, till death do us part. - Kenshin "Shinta" Himura - Rurouni Kenshin