This page lists the six Big Octos that you can find in the Great Sea, along with the number of eyes and the treasure that you get from each one.

1. Location: Two-Eye Reef (one square south of Southern Fairy Island)
   Number of Eyes: 4
   Treasure: magic meter upgrade
2. Location: Diamond Steppe Island (one square northwest of Outset Island)
   Notes: The hookshot is required to get onto Diamond Steppe Island.
   Number of Eyes: 8
   Treasure: 100 rupees
3. Location: Private Oasis (one square northeast of Southern Fairy Island)
   Number of Eyes: 8
   Treasure: 100 rupees
4. Location: Fire Mountain (one square northeast of the Tower of the Gods)
   Number of Eyes: 8
   Treasure: 100 rupees
5. Location: Seven Star Isles (one square north of Dragon Roost Island)
   Notes: If you encounter kargarocs here, grapple them for their golden feathers, then either kill them with the boomerang or just sail away.
   Number of Eyes: 12
   Treasure: piece of heart
6. Location: Tingle Island (one square southwest of Windfall)
   Notes: Watch out for gyorgs (sharks).
   Number of Eyes: 12
   Treasure: piece of heart