I'm going to assume you can't flashback spells for free with Omniscience since it's an alternative cost but just wanted to check to be 100% sure.

Edited Nicholas Cummings (March 19, 2015 04:37:51 PM)

Both Flashback and Omniscience provide alternative costs to cast your spells, and it's true you can't use two different alternative costs on the same spell.

However, there's also another reason you can't flashback spells for free with Omniscience: Omniscience only allows you to cast cards from your hand without paying their mana costs.