As you can see from here the IW is not OOT. At the end of the IW and at the start of LTTP Ganon has the whole Triforce to himself, inside the Dark World. At the end of OOT Ganon ONLY had the Triforce of Power when he was sealed. In LTTP one of the maidens says knowledge of the the Golden Land's whereabouts was lost over time, but Ganon rediscovered it, thus triggering the IW:
In OOT knowledge of the realm that holds the Triforce was not lost. Zelda and the Royal Family knew, that's what the Temple of Light was for and they knew it's secrets, that's why Zelda threw the Ocarina to Link, she knew what it would do. Even Ganon at least suspected this--he just didn't have a way to get inside, until Link helped him, indirectly of course:
(Maiden in Swamp Palace)...The Triforce will grant the
wishes of whoever touches it,
as long as that person lives...
That is why it was hidden in
the Golden Land. Only a select
few were told of its location,
but at some point that
knowledge was lost...
The person who rediscovered
the Golden Land was
an evil thief named Ganondorf.
Luckily, he couldn't figure out
how to return to the Light
GANONDORF, TALKING TO LINK TELEPATHICALLY (centered):
Geh heh heh! Excellent work!
As I thought, you held the keys to the Door of Time!
You have led me to the gates of the Sacred Realm...
Yes, I owe it all to you, kid!
(long, roaring laugh)
I placed FSA after LTTP, since the Dark World is around in that game, and many of you know that the realm wasn't called the Dark World until after Ganon touched the Triforce. (Thanks to Octorok, Zol...Fyxe for pointing this out to me) Although Ganon is dead at the end of LTTP, he was apparently reincarnated in FSA. In LTTP Aghanim mentions something about the tribe of evil and in FSA a maiden mentions that Hyrule was once invaded by a "tribe of evil", maybe she's referencing the events of LTTP and most likely the 'tribe of evil" is just general for any evil force or a term used for Ganon and his followers.
At the end of FSA, Ganon is trapped inside of the Four Sword. Somehow he is broken out, maybe by Twinrova, who will later play a role in OOT. Ganon continues his search for power, and soon Hyrule is enveloped in war amongst its races. A Hylian mother tries to escape the ravages of the war, and on her last breath entrust her baby with the Deku Tree. This war will later be referred to as a "fierce war" by the Deku Tree and Zelda Fandom will coin it as the "Fierce War" (FW). We all know what happens in OOT (sort of) so I won't elaborate on it.
I'd go on, but I'd rather stop and see what others think of this, and please speak freely, I'll try to refrain from biting your heads off, if you have a problem with this.