This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Odin is in the basement of Baron Castle. At the beginning of the game,
the area is off-limits to Cecil. When Cecil returns to Baron through the
Old Waterway, the passage leading to the basement is magically sealed.
Once Kainazzo is defeated, however, the seal will break and Cecil can go
into the basement.
When you first meet Odin, he will probably ask Cecil to go to the Land
of the Summoned Monsters before returning. If you just defeated Kainazzo
and came down here, keep in mind that it's going to be a very
loooooooooong time before you get that far in the game.
When Edge finds the Falcon Airship, and Cid has coated the ship so it
can fly over lava, you can fly to the Summoned Monster Cave, which is on
an island near the middle of the Underworld. At the very bottom of this
cave is the Land of the Summoned Monsters. When you reach this point,
before you leave, make sure Cecil talks to Queen Asura and King
Leviathan. Also, read all the books in the library before continuing.
(We mean it! They're important.)