He didn't.
They used magic to bring him back to life.
Unless they're playing with time, Oliver's has been in the snow shirtless for almost four days.
That alone should kill him.
The Character, Tatsu, in the comics has a sword that carries the soul of her dead husband, which is magic power of her sword, rather than her own, but it does lay her side by side with necromancy.
(I'm laughing)
What if her hut is built below/near a Lazarus pit, and the pits properties are leaking into her (well) water supply?