Olaf has the potential to be the most powerful champion in LOL.... Olaf is a berserker, and needs to be treated as such. he is an all in champion who crushes his enemies or dies in the attempt. In order to be strong with olaf you must understand how and when to unleash the berserker.

Olaf thrives in the heat of battle. the top lane may aswell be Lokfar, the homeland of Olaf.
the runes you take are pretty standard.

flat armor
magic resist per level
attack damage

however with olaf his quints should be.......LIFESTEAL!!!!
why...oh why lifesteal do you ask?? he already has a wriggles in his build...and vicious strikes gives even more lifesteal!!

because Olaf needs sustain both in lane and surrounded by enemies in a teamfight. Olafs passive grants him increasing attack speed at low health. more attack speed equals more lifesteal....

combined with wriggles lantern, olafs passive, vicious strikes and his ultimate which prevents CC.... a low health tanky olaf can be unkillable.

Olafs abilities dont require help from items.
vicious srikes gets some extra AD from health
Undertow gets some extra AD from....AD

the most important thing for Olaf is being able to survive teamfights. put a minimum of 21 points in defense.
make sure to get indomitable...a dmg reduction of 2????? seems like nothing right. this skill is what makes you early game laning phase much easier. when you attack a champion and take 50 hits from his minions, this skill will reduce every hit by 2.

Olaf is a bruiser.
your core build is

wriggles lantern - for lifesteal armor and the passive damage to minions. (vicious strikes gives spell vamp which will heal you for a part of that magic dmg)

Mercury treads - Your ultimate cant be used at all times. Olaf needs to be able to keep swinging or he dies. also you will need early game MR when the mid lane comes to try and stop your steamrolling of top lane.

phage and frozen mallet - When you initate a teamfight you need the health to survive the burst and the slow to prevent your helpless victim from escaping. a victim who cannot escape is also a steady tream of lifesteal to keep you alive.

Atmas impaler - Olafs vicious strikes gain AD from health. so does atmas impaler. you may aswell get twice the bonus DMG for your money!! Also you gain a huge chunk of armor and 18% critical chance. thats almost 1 in 5 chance that olaf will be hitting like a truck. which equals even more lifesteal

after these four items the last two are situational. I like wits end for the attack speed and stacking MR. it benefits olaf immensly when at low health. I also like sunfire cape for the extra tankiness....besides if im going to charge in the middle of the enemy team I may aswell deal some aoe magic DMG

Your first skill should be undertow. you will use this skill to farm, poke and chase. wait until the enemy flees from you in terror, then throw the axe. you cooldown is reduced by 4 seconds if you can pick it up. This means that your victim can be permanently slowed.

Your second skill is reckless swing....this is your most important skill. it deals true DMG and should be maxed as soon as possible. because of your lifesteal runes and items. the health you lose when activating this ability is negated... Also this skill uses no mana. So even when out of mana Olaf can still deal massive DMG.

vicious strikes is your third skill. because I build Olaf tanky and I have lifesteal runes I can max this ability last. I prefer the extra DMG and utility of reckless swing and undertow during the laning phase... However during late game teamfights vicious strikes is what makes Olaf so unkillable. with 6% lifesteal runes, 12% lifesteal from wriggles lantern and 21% lifesteal from vicious strikes you have a massive 39% lifesteal. combined with his passive and Ragnarok, your sustain is frightening. when using this skill in a teamfight wait until you have taken the enemy burst. that way you can benefit even more from your Passive

Ragnarok is your ultimate. It grants passive armor penetration (which is why i dont bother with Armor pen runes). most importantly it grants massive armor and magic resist and completely prevents CC..

ragnarok is what makes olaf a true berserker. you can pop your ultimate and walk straight through the enemy team picking your traget. there damage is reduced by your defense masteries, tanky build and bonus armor and magic resist from your ultimate. you use the constant slow from undertow and frozen mallet to prevent the enemy carries escape. while the enemy team desperately tries to save there carry you press vicious strikes and with your passive and bonus damage and lifesteal you heal back more and more health the closer you are to death.

now the enemy team has blown all of there cooldowns on you......and you are still standing!!!
you and your team mop up the rest and on to win the game.

when it comes to summoner spells use what you please.

teleport is very helpful when you are top lane. it also can help you get to teamfights that would otherwise be too far away.

ghost also combines very strongly with undertow and ragnarok. No matter how hard they try they cannot stop... or escape Olaf.

Build Olaf as the tanky bruiser that he is and you will be met with success. Olaf is a frightening presence on the field and has a tendency to be instantly focused.

If you can survive the burst.....they will feel the wrath of a berserker!