The effect of wither
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.