you get the 7th one .. i guess you looking for that one, when you get one item of every item in the game. There are only seven Orichalum + in kingdom hearts :]
The ultima weapon is the best but requires the hardest process. For example finding Orichalum's. Other than that Fenrir is the strongest which is rewarded for defeating Sephiroth.
you get it in kingdom hearts final mix after defeating sephiroth
By defeating Jafar.
You can get it by defeating Sephiroth in Radiant Garden.
YOu can find it anywhere--from mostro to hallow bastion. Just look around in some treasure chests and you will find some.
Orichalum can be found in chests or by the Bulky Vendor (rare heartless that doesn't attack) best way to find all the chests is to use a guide, found on many websites. one is faqs.ign.com/articles/701/701619p1.html
By defeating nobodys
either the little blue dude in the underworld. Or Hades. hope you were talking about kingdom hearts 2
Orichalum- Found in some treasure chests and is also given as a reward for certain tasks- Behemoth (End of the World)- Invisible (End of the World)
Power Crystals can be found by defeating Wyverns in Kingdom Hearts and Morning Star in Kingdom Hearts II.
The Aeroplane Heartless can be seen in Kingdom Hearts 2. Locations: Timeless River, Pain and Panic Cup, Pain and Panic Paradox Cup, Hades Paradox Cup.