Since giving a minion to your opponent does not count as summoning, the combo involving Spiritsinger Umbra into Treachery into Octosari into Treachery does not trigger the Deathrattle of Octosari for your opponent.

Edit: This combo also does not work because reduced Mana costs are reset after Plot Twist. :(

Below, a new combo with the same effect is introduced. It has the advantage of being less expensive and requiring less cards.

Main combo pieces:

1. with 4 of Dollmaster Dorian, Plot Twist, 2x Treachery and Mortal Coil in hand, play Emperor Thaurissan (reduce cost by 4 Mana)
2. draw your whole deck, do not play the remaining combo pieces
3. play Dollmaster Dorian followed by Plot Twist
4. cast 2x Treachery on the mini copies of Octosari and Baron Rivendare
5. play Mortal Coil
6. watch your opponent's deck (and health) melt