With this build Olaf became very strong farmer you can kill whole waves only with few Undertows.
Its very important to farm at start, after you take your Emblem of Valour just go on your lane and farm as most as possible. You can use your Undertow for very fast farm but if you want to save some mana try to farm only with Vicious Strikes like that you can get nice life steal so the enemy wont take down your hp easy with harass. But if they bring you low hp they could jump with your attack speed runes and low hp you can get sick attack speed on low lvl but jump only if you got saved mana so you can chase them down with Q,W and if you got ult.
And attack speed really helps at your farming at start youll start with around 1 attack per second at lvl 1 if you start with Dagger (If you dont want Doran Shield you can change it with Dager) , if you make first blood and enemy focus you and take you low hp dont run if there are now big difference in hp .(Remember at 1% hp you take 99% bonus attack speed + 40% more from runes + mastery + 15 % Dagger ( if you like want to change your Doran Shield )

Item build is based on HP,Attack Speed,Armor Penetration,Life Steal,Movement Speed,Slow,Attack Damage

I just like attack speed and you get armor pen,armor,magic resis,dodge,ms after dodge,crit chance,attack speed,health

Attack speed ! Makes you hardcore !



Other you can get:

Armor pen
Armor
Magic resis
AD
Movement speed
Crit chance
Crit Damage
Health
Dodge

Main S.spells : Ghost,Exhaust
Ghost - help chasing down enemy running away(very effective with ult)
Exhaust - help for first blood , chase enemy , slow some1 down to save your self or mate or just to kill him or if there is a fed enemy like Jax who kill you fast use it to lower his crazy damage.

Other options :
Flash - good for last hitting running away enemy before he get too far away(for most heroes maybe flash is better but for Olaf he need more Ghost)
Ignite - if you are lazy to chase low hp enemies
Telleport - for deffending towers or backdoor or faster way to get on your lane
Cleanse - you got ult but still usefull
Heal - saving your self,mates and if you are noob

NEVER TAKE ! :
Clarity - mana.... who cares
Revive - Troll
Rally - Uninstall your game now !!
CV - For what ?

The big attack speed bonus make him really good jungler.
To jungle first go to golems and take your abilities just like i show before you need main spell Untertow and secondary Vicious Strikes.
But in here the build changes first you take CLoth Amour and 5 hp potions then after you get golems,ghosts and wolfes go base and take Vampiric Scepter go to blue buff and kill it then go to red and then you can go gank. Then whe nyou get enough gold go base and take Latern and Boots of Speed after that get Phage then if you want get Starks and Trinity Force and continue the build like i show . ( if you want you can sell Latern and get your full build like non jungle and other way to start is to get Vampiric Scepter as first item and go golems,ghost etc.)
Im sure that if some pull Blue buff you can make it at start too (didnt test it)

PROS
Great Farmer
Carry
Sick slow
Sick damage
Sick attack speed(more than 2 attack per second with full build at 100% hp)
Nice health
Good survivability(mostly in team fights and used ult and W)
Incredible pusher
Fast movement speed
Good harass ( Untertow )

CONS
Can be kill fast from ranged hero like Caitlin when his ult on cd
Tankish but not immortal

In ult - Immune to all CC
On low hp , used W and Ult makes him almost immortal in fight