Omnislash's damage type is pure. There is no reduction for the 3/5/8 hits. Each of the specified 3/5/8 hits deals 150-250
exactdamage. However, Omnislash works in mysterious ways. If you have a high enough attack speed, in between those slashes Yurnero will strike his regular hits to the opponents. These are not the specified 3/5/8 hits, but his normal attack. These are physical damage. If you have basher, these "extra" hits might bash; if you have S n Y, these hits might proc "Main"; if you have Skadi, these hits will definitely slow.
Brain Sap too is Pure damage. As far as I know, Mana Void and Scythe are Magical, meaning they will be reduced by Spell Reduction.