Predator on Olaf allows you to hit the go button on ganks and not let up. With the CC immunity from [R] you'll be hunting down anyone who gets in your way.
WhyCheap Shot
Cheap Shot allows your to repeatedly deal True Damage to whatever target your attacking thanks to the perma-slow from [Q] and Righteous Glory.
WhyZombie Ward
Zombie Ward on jungler provides massive amount of vision thank to your natural deep warding to keep track of the enemy jungler, as well as clearing of enemy wards with Sweeper/Control Wards.
WhyIngenious Hunter
The item CDR from Ingenious Hunter allows you way more uses of Predator and Righteous Glory for those unstoppable ganks.
WhyNimbus Cloak
Nimbus Cloak's burst of Movement Speed synergizes well with Olaf's [R] playstyle when looking to chase down a target.
WhyWaterwalking
Waterwalking's jungle control power and mobility is huge for contesting crabs and dragons.