i've won so many games as olaf you'd be a fool not to take my advice.

self explanatory. early game you're going for as much damage as possible.

also explanatory. you want movement speed though. otherwise the non-offense mastery points can be put wherever.

tricky. you want to build damage>attackspeed>lifesteal>utility. what i mean by this is you might not want boots for a while, you may want zepher mid/late game for speed and atk-speed. your primary concern is getting lifesteal and attackspeed though.

there's three ways to play olaf. undertow the hell out of your enemy if he's squishy, and follow up with E as often as possible. if your getting overpowered, this is also a valid strategy, so focus accordingly.

assuming everything went well, you want to go, W,q,e, then focus on W and E. you make your money by repeatedly using E in a battle. the more true damage the better. the more attackspeed the more true damage. ;]

you can use ignite, but i prefer ghost and exhaust. exhaust will save you more then using ignite in most cases. just trust me, movement speed is OP.

basically you're going to want to E as often as possible using your attack speed to your advantage. three words of advice with JUKE JUKE JUKE. it's best to let your enemies chase you to ensure you have enough room to chase them down with undertow. ideally you want to get 2-3 undertows in when chasing someone back.

also, use w at around 70% health, assuming you're losing less then 10% per second (assuming you know how to gauge your survive-ability, lol).
watch out for the jungler until level 6.

try not to use your ult unless you need to. but if you need to, use it when you're ABOVE or at 60% health.

the pro's to my build; great for team fights, you WILL chase people down, you WILL take names. there are too many pro's to list because this is olafs build. (especially with sheens as 4th major item)

cons; duo top can give you some trouble, so you'll have to outsmart them, juke them, and hug the tower. don't worry about losing the tower before 12 minutes if you're against duo top and your junglers not around. just focus on living and avoding tower dives.

other cons; low health early game

you're going to want to use undertow once for each group of minions, but only if you can hit 3+minions AND the enemy champion. use E all the time when it's free.

it works well to run away, and when they begin to chase you, throw undertow at them. normally you'll juke and get them, openning you up for more undertows or a kill.

it's important to communicate with your jungler. you can't let yourself get like 2-3 levels behind someone and come back without your junglers help.

after you get your first tower, gank mid as often as you like to help the jungler; even if he never helped you because you're a badass bloodletting barbarian who can't stop killing.

undertow people to help your team in chases. getting boots of swiftness and zepher make you impossibly quick- add undertow and you're a shiny golden god.

if you're dying slowly and you hit 50% health with w and R up, use them and pop ghost. if you're near a squishy champion or a tank that you can outlast you WILL win. olafs key is being stronger then he appears because of his passive. utilize his passive. his passive wins you early game. his passive gives you first blood every game.