90 to 120 minutes? So far I've played Oblivaeon 5 times with a few different groups, and one thing is certain: no game of Oblivaeon has lasted less than 2 and 1/2 hours, unless we gave up. Two of the games lasted 3.5 - 4 hours.
It's certainly not a deal breaker, but it does bring into question how often my groups will play it, considering how much of a time investment it is, compared to a few normal games of Sentinels. But, I've found there are a few things you can do to greatly simplify and speed up a game of Oblivaeon.
1. After your hero is incapacitated, don't change decks until you've played all of that hero's variants.
2. Don't use any overly-complicated environments, such as: Realm of Discord, Wagner Mars Base, Omnitron 4, Celestial Tribunal, Tomb of Anubis, Madame Mittermeir's, Magmaria, and Temple of Zhu Long. There's just too many complicated cards in them.
3. Use the "Primary Objective" shield.
The other shields are a bit random, with random factors controlling the game more than the heroes. We'd often wind up sitting and waiting, unable to do anything to bring down the shield. Primary objective is at least something you mostly have control over, helping keep the game moving.
4. Don't use the "Sanction" Scion.
It's not even a very hard Scion, but it has a lot weird steps to follow and keep track of. I'd either omit it, or (to preserve game balance) make it the last Scion to enter play each game.
5. Vassal Aeon Men only.
This one is admittedly the biggest change, but hear me out. Aeon men are the fiddliest, most cluttering part of Oblivaeon games. Aeon Vassals are the most statistically average one, not too easy and not too hard.
Just keep an Aeon Vassal card between each battle zone, and every time cards from the Aeon deck are to be played, you just put a 6 sided die, set to 5 (for its current health), to the appropriate side of the card. It's a lot less to keep track of.
Now, in order to keep game balance, when they're killed by Oblivaeon, instead of resolving the effect at the bottom of their card, THEN draw a card from the Aeon deck (the regular deck, including all types of Aeon men) for each Aeon man killed, and resolve that card's "when destroyed by Oblivaeon" effect. This will give you a more balanced/random effect, without slowing the game down during normal villain turns.
If any of these seem to make the game a little too easy (#3 admittedly might), then just fight him on advanced mode. I'm not trying to make Oblivaeon easier, I'm just trying to bring game length down to 2 hours.
Good luck, and have fun everyone!