This is my Olaf build that I use consistantly throughout my LoL career, I find that he is rather enjoyable to play as and is fun throughout the match,
Note: I only use this in 3v3 matches, so a 5v5 may differ slightly.

For Runes, I go full out Armour Pen Marks for the early game Armour Pen and to maximise early game damage,
For Seals, I go full out dodge for that little bit of evasion every now and then,
For Glyphs,I go for Attack Speed, because Olaf needs a bit more Attack Speed at the start of the game.
For Quints, I go Flat Health, for some survivability.

For Masteries I go a 21/9/0 build, getting increased damage in Offense and getting reduced Damage and more armour/magic resist in Defence.
This way, You've got some decent damage tanking capablities and have a high damage output.

First up, I get boots of speed, this makes getting to the bottom bush in Twisted Treeline much easier and then you can go for first blood, the health pots add a bit of survivability in the lane and can help you focus more on farming then having to b back all the time. When you've got enough gold, b back and grab Beserker Greaves, and the Ruby Crystal and return to lane, keep farming minions and ganking other lanes to get enough gold to upgrade to a Phage and then eventually a Frozen Mallet. Soon after, getting Warmogs will give you more Health to fuel your Vicious Strikes Ability, and getting Atma's Impalier will deal more damage because of the Health Boost from the Warmogs. The game is usually over by the time I get my last item, but if the game goes on longer, I get items depending on what my opponent is, If they're mostly Ad then i get Thornmail, If they're more Magic based, I get Force of Nature. The last item for a bit of extra tank is Randiumins Omen.

The way I choose my skills is get Undertow first, because the long ranged slow is good for getting first blood and good for a little bit of harrasment, you have to be careful though, because you may run out of mana. Vicious Strikes is up next, and I level it up whenever I can, because it's easy to 1v1 someone when your one low health because of Vicious Stirkes Life steal Effect, and because of Olaf's passive, Beserker Rage, which increases his Attack Speed depending on how much Health Olaf is missing. Next up I get Reckless Swing, The true damage effect of Reckless Swing can easily take down Tanks and Squishy characters because the damage is so high. I get Olaf's Ultimate Ragnarok whenever it's avaliable.

For Summoner Spells, I get Flash and Exhaust, flashing being for the Escape potentail, Exhaust is to stop those kills from getting away and to reduce damage.
If I don't choose Exhaust, then i go with Ignite, for that little bit of extra damage.

For Farming, you want to focus on last hitting minions a lot during the laning phase. With Olaf's High base damage, you can easily get at least half the minions in your lane.

If your laning with someone else try and co-ordinate mia calls and gankes effectivly.

For Teamwork, you'll want to be on the lookout for potentail ganks, I.E people that have over extended, or are chasing your team for a kill.

During a team fight, Olaf can be devistating, as long as he's not focus and stunned, Activating Ragnorok in a team fight will makes them waste there stuns on you, thus allowing a easier time at defeating them. Olaf's damage is so high, you can easily take down squishys first to minimise damage to your team.

Pros:
High Damage output at late game and mid game.
Hard to die in a 1v1 due to his Combo of Beserker Rage, Vicious Strikes, and Ragnorok.
Easy to get kills with.

Cons:
This build is a bit expensive, and requires lots of farming.
You'll probably get ganked alot, because of the damage you pull.
Can't take on more then 2 people at a time at high levels.

Olaf is an extremly fun champion to play, but may be a bit hard to grasp at first, he is one of my favourite champions and is really easy to be succeseful at if you play him right.
The high damage and tank items are good for him because he can survive the damage and deal it back really well and is a real Riot (games) to play as.

Olaf would like to thank you for taking the time to read his build and would be happy with any feedback you may have, Thanks very much, and go Berserk on your enemies.

Beware The Beserker, He will come forth with a fury and rage that is unequaled, He will not stop untill his target is either running away screaming in fear, or dead at his feet, with two massive axes sticking out of there body.