If Olivia Voldaren is enchanted with Fists of the Demigod and you activate the ability to deal damage to a target creature, does this damage come in the form of -1/-1 counters? If it does, and the target is a Spirit en-Kor, can the Spirit en-Kor redirect the -1/-1 counter?
Yes, Wither applies to all damage, not only combat damage.
702.79a Wither is a static ability. Damage dealt to a creature by a source with wither isn’t marked on that creature. Rather, it causes that source’s controller to put that many -1/-1 counters on that creature. See rule 119.3.
Yes, Spirit en-Kor can still use its ability to redirect the damage. Olivia Voldaren is still dealing damage, it's just that the effects of that damage are different than normal, so the Spirit's ability can be used. And when the damage is redirected, the source of the damage is still Olivia, so Wither still applies.