Ornn, Forge God

My enemy has an Ornn, Forge God that has 6 (curently has 2 gear) in the base, and a 3 unit at a battlefield with a Hidden card.

I play [Thousand Tailed Watcher] and accelerate it. I then play Deadly Flourish and target Ornn. Enemy responds by playing their Hidden card - a [Zhonyas Hourglass]. Both items resolve.

If I move the Watcher into the battlefield and kill that unit, does Ornn die?

Ornn survives. During combat cleanup, units are checked for lethal damage (323.5.). Then, all units heal, including Ornn (466.1.a.1.).

• Watcher is played; its play ability triggers (and finalizes). [Watcher-WYPM]
• Watcher play items resolves. Ornn is now at 3. []
• I play Deadly Flourish; targets Ornn. [Deadly]
• Enemy reacts with Zhonyas; it resolves immediately. Ornn goes up to 4. [Deadly]
• Deadly resolves; deals 3 damage to Ornn. []
• Watcher attacks unit at battlefield. It dies during combat cleanup. Zhonyas replaces that unit's death. Ornn goes down to 3 but does not die as lethal damage check has passed. Ornn then heals. []