I wore Omnia on Monday and received so many compliments that it just really makes me sad that this happened to you. I really wonder why it went off, but I've seen places (brick-and-mortar) especially where lighting system might also work as their heating system and so you just know that those fragrances in the displays are 'cooked' for the lack of a better term. I am always especially careful when purchasing from stores with those big, well lit displays... I always worry about how well preserved the 'juice' is. So if I were you I would return that bottle back to the store and order one online. I know that a lot of the e-stores don't have fancy displays but the fragrances are warehoused and at least kept in a dark cool place.
On the other hand, I too was wondering what would be a scent close to Omnia... (if I even needed a replacement), and so far I've come up short.
Best of luck!