How about removing Mana as a Resource for Olaf?
It's a quite simple change that would fit thematically.
Remove the mana cost from his Vicious Strike and give him an "Axe" resource that he has to manage for his Underthrow.
Basically, give him an "Axe" resource bar that fills over 12/9/6 seconds at level 6/11/16.
If Olaf throws an Axe, the bar is consumed and Underthrow gains power depending on how much of that bar was depleted.
Underthrow no longer slows and deals less damage than live now.
But if consume 0 - 49% of your Bar, it doesn't do anything
If you consume 50 - 69& of your bar, Underthrow Slows (just like it does now)
If you consume 70 - 89% of your bar, Underthrow deals 50% Bonus Damage.
If you consume 90 - 99% of your bar, Underthrow Slows, deals 50% Bonus Damage and gains 33% Bonus Armor Penetration.
If you consume 100% of your bar, all the Previous Bonues are Applied and Doubled.
Olaf will only generate the bar when Underthrow is off cooldown. Grabbing an Axe now instantly refreshes Underthrows cooldown.
This gives Olaf more depth with a different mechanic that affects how good his Underthrows will be, which is his core ability anyway.
If you just spam Axes, they will be weak and useless. If you let them grow, they will be devastating.
This also provides more counterplay as Olaf isn't able to permaslow you anymore until very late game.
