Can you obtain the Omnipotent Orb in this game?

[Fluffal Kyurem]

And if you can, is it usable in the Margaret boss fight?

[John Johnson]

If you use it I'm pretty sure she just spams 9999 megalodan's, just use absorb skills

[Truthful Sentiment]

You can indeed obtain the Omnipotent Orb. From my understanding the Reaper will drop it once each character's strongest weapon has been obtained, and a set of the strongest armour for each of them as well before it begins to drop them properly; although you may be able to force it to drop one if you fight it with a party that has their weapons already, but not having them for the whole group, as I had one under similar circumstances.

For your other question, no. Any accessory that nullifies damage will force Margaret to instantly kill the whole party. Personas that have skills to the same effect won't trigger this, though.

[Fluffal Kyurem]

Originally posted by Truthful Sentiment:

For your other question, no. Any accessory that nullifies damage will force Margaret to instantly kill the whole party. Personas that have skills to the same effect won't trigger this, though.

That does make me curious as to how long you could survive though. Naoto's ultimate skill "Shield of Justice" would reduce it to just 1 Insta-kill each turn. Rise can block it once. Everyone can endure a mortal blow once and Rise could also self-rez you. Enduring Soul can resurrect you an additional time. So probably 4-5 turns.