Does this Combo Work?

Protective Spirit + Offering of Blood

With a Sac Necro OoB could be useful but 20% is a bit high especially if there are other sacs like Dark Pact & Touch of Agony. So does anyone know if Protective Spirit cuts the sac on OoB in half?

No, with Protective spirit up, you still sacrifice 20% of your max health to Offering of Blood.

Life sacrifice and actual damage are two different things. Prot Spirit just works on damage.

Aura of the Lich reduces sacrifice costs, as does multiple Superior runes.

PS only works on damage taken from ATTACKs or SPELLs. Meaning the use of anything that isn't an attack or a spell will bypass PS. Including skills that are labelled SKILL.