In the new patch v 1,0,0,107 olaf has gotten nerfed but not that much:
Undertow now deal physical damage instead of magic damage
Vicious Strikes health scaling changed to 0.5% at all ranks from 0.3/0.6/0.9/1.2/1.5%
Vicious Strikes base damage changed to 10/17/24/31/38 from 12/18/24/30/36
Ragnarok damage reduction reduced to 20/30/40 from 25/45/65

Even with these nerfs u can still get pentakills (100% tested). And u will be still so hard to kill that it will get annoying to some people

Pro's:Very quick jungler (better than Warwick)
Good amount of life
Good damage output
LOTS OF LIFE STEAL!!!
Very fast/ Great chaser
Can mess people up in teamfights
Cheap to a point. (The late game items are expensive but you have a lead with cheap early items.)

Con'sWithout your ult, you are defenseless/helpless child in a teamfight
Doesn't dish out Amazing damage like Ryze or Yi
Low magic defense
Your not as amazing at jungling as Fiddle....
Your late game items are expensive and you probably will never get them all.

ABILITIES:
Q - Undertow - Olaf throws an axe into the ground at a target location, dealing damage to units it passes through and slowing their movement speed. If Olaf picks up the axe, the ability's cooldown is reduced by 6 seconds.
Olaf throws an axe to a target location, dealing 65/110/155/200/245 (+) physical damage to units it passes through and slowing them by 24/28/32/36/40% for 2.5 seconds.

If Olaf picks up the axe, the ability's cooldown is reduced by 6 seconds.

A great chase spell just run, undertow, pick up axe, and throw again i dont max first because slow is now 24% not 16%

W - Vicious Strikes - Olaf's attack damage is increased, based on his health, and he gains massive lifesteal and spell vamp.
For 6 seconds, Olaf's attack damage is increased by (10/17/24/31/38 base damage plus 0.5 of his max health). While active, Olaf gains 9/12/15/18/21% lifesteal and spell vamp.
Great spell i usually max this second because i dont need it pretty much in the beggining but i still put 1 point in it before lvl 6.

E - Reckless Swing - Olaf attacks with such force that it deals true damage to his target and himself.
Olaf deals 100/160/220/280/340 true damage to his target and inflicts 40/64/88/112/136 true damage to himself.
Kiss ur tanks goodbye reckless swing is here. 340 true dmg is awesome tanks stand no chance (lets not talk about squishies :D). I max this first so u can get enemies down in 5 sec its the greatest spell of his in my opinion.

R - Ragnarok - Olaf is immune to disables, has increased armor penetration and reduces incoming damage.
Active: For 5/6.5/8 seconds, Olaf is immune to disables and reduces incoming damage by 25/45/65.

Passive: Olaf has 10/20/30 increased armor penetration.

No comment :D

Passive - Berserker Rage - For each 1% of health missing, Olaf's attack speed is increased by 1%.

*cough* *cough*


item build:
1. breserk boots: u can take merc if they have a lot of cc but i find these better.
2. Wriggle: its a great item, when ure low just hit some creeps and ull be back to 100% hp in no time and u can easily solo dragon with this item
3. frozen mallet: its a great item for olaf it gives hp, dmg, slow and it will scale well with his passive with frozen mallet and undertow u can chase anyone.
4. starks fervor: more att speed on low hp more lifesteal it will keep u alive longer.
5&6 item u can take either: madred: its great to get to those beefy characters.
black cleaver: if they have armored tanks. Bloodthirster: for more life steal and dmg. Last whisper: for those teams who have really armored tanks ex. rammus.

Summoner Spells:
Cleanse: to get rid of the annoying blind the ultimate doesnt remove
Exhaust: its great to have exhaust with u so u can slow and blind enemy melee or ranged dps champions. Example: Sion. His annoying ult keeps me from killing him but he can destroy me. So i just pop exhaust on him when he ults and it usually gives me the time to kill him.
Ghost: u can take ghost instead of cleanse so u have exhaust ghost combo to escape or to chase ur enemies

ATTACK COMBINATION:

Just use ur ultimate if they are standing and trying to kill you, then you are immune to stuns, slows and so on. Well when you have taken your ultimate just spam QWE when you can. In every case u will kill your opponent.

Early game - You should farm minions and not try to get kills a lot.

Mid game- You should try and gank a little bit but you can lane as long as you want. Try and get lizard and golem if you're running out of mana. You should focus on your frozen mallet if you already have your zeal and emblem of valor. If you're ganking dont be afraid of running in, if you think they may run away get your ult and ghost, throw an axe at them and you should have lizard buff, so it will be really hard for them to run.

Late game- You should have most of your items right now. If you need to start teamfights go ahead, but try and let the tank initiate, if you're going to initiate pop your ult, ghost and go in. Kill 1 or 2 and if you can, run. Don't be too aggressive, sometimes you can handle a 3v1, if you're doing good but if you're too aggressive you will only run into a fight you can't win.


5000 CHARACTERS FINALLY WOOOOOOOOO :D

NOTE: i copied some text from different builds so if u see it dont get road raging :D

[Frozen Mallet]