Do you have to pay the commander tax if you have an Omniscience in play?
The commander tax is only applicable when casting the commander from the command zone.


  903.8. A player may cast a commander they own from the command zone. A commander cast from the command zone costs an additional {2} for each previous time the player casting it has cast it from the command zone that game. This additional cost is informally known as the “commander tax.”


But Omniscience says


  You may cast spells from your hand without paying their mana costs.


(emphasis mine)

If your commander has been bounced back to your hand, you may cast it for free, thanks to Omniscience.

Side note: it's rather hard to have an Omniscience in play if Zo-Zu the Punisher is your commander. I thought there was a red card which lets you exchange control of permanents (maybe after a coin flip) but I'm not sure...