Pros:
-Very strong in early-game.
-Good chaser with his Undertow.
-Very durable, dies very hard in late-game.

Cons:
-Has mana problems in early-game if you play aggressive.
-No natural escape mechanism.
-Dependent of item/gold in the game.

-Teleport helps you get back in lane very fast after recalling, so you won't loose much experience. You can also use it defensively, to protect a turret from being destroyed, or offensively, to destroy a turret, to gank an enemy, and much more possibilities.

-Why Ghost and not Flash?Because Olaf is tanky, and Ghost is more useful due to the lower CD, and it covers a better distance in total. And Flash was nerfed, lower range, longer CD, so i prefer Ghost :), but if you want to go with Flash, that's just fine. Your call.

I go for Armor Penetration on Marks and Quint's because this is a must, especially on champions like Lee Sin, Olaf, Wukong, Graves and so on. From the runes you will get a nice 25 Armor Penetration bonus. That means that 25 of your opponent's Armor is useless.

Armor Seals and Magic Resist Glyphs gives you early game sustainability, and makes you more durable in lane. You can take either per level or flat, but I prefer the ones per level because it gives you more Armor and Magic Resist at lvl 18.

On masteries I take 9-21-0. 9 points in Offense for the Armor Penetration, Attack Speed, and the Ghost point.

I take 21 points in Defense for maximum protection. This fits well with your Armor and Magic Resist runes. The Damage Reduction comes in handy on Olaf, because you want to be a bit more tanky than other champions, especially with that HP that you're building.

-This is a very good item for Olaf. Gives you some Armor bonus, Damage, and some nice Lifesteal. But it also helps you farm much much easier because of it's passive, and that is a good thing for Olaf, since he is a very item-dependent champion.

-These boots give you some extra attack speed, that you surely need, especially with your lifesteal from Vicious Strikes, and the attack speed from Berzerker Rage when you are low on HP.

-Frozen Mallet is one of the best items for Olaf. It gives you HP and some AD, plus the cool passive which slows your target's movement speed, and lets you chase your enemies more easily. Also the 700 HP that you get from it, really helps you when you activate Vicious Strikes

-Armor, Attack Speed, Damage. All that Olaf needs. Plus the passive gives you a nice damage boost, especially against enemies that will start stacking health.

-The HP bonus will help you take more damage, it fits perfectly with Vicious Strikes, and you can really spam your Reckless Swing. It's like this item was made especially for Olaf.

-This item...this item is just perfect for Olaf. Not only that it gives you 18% Critical Strike and 45 Armor, but it makes a perfect combination with HP items like Warmog's Armor/Frozen Mallet. At the first look, 2% doesn't look like much, but with a total of 4-5k HP, it adds up to 80 Damage.

Shoes

-This is a very good choice. In almost every game you will face a pretty balanced team composition, with different forms of CC. Also the 25 Magic Resist is very useful because the enemy team will want to take you down before you get too tanky for them to handle.

-Dodge and Armor. A perfect combination against champions like Tryndamere, Xin Zhao, Master Yi. If their team is made up of 2 AD champions or more, just take them if you feel like you need some extra Armor.

-Not seen on olaf that often, but if you don't need Attack Speed, Armor or Magic Resist, buy these, since Olaf is a little slow. These shoes will help you to chase your opponent or to escape him.

Offensive

or -After you get your Phage, decide weather to get Trinity Force or Frozen Mallet. I honestly prefer Frozen Mallet because of the HP bonus and the passive that slows your enemy. Also Trinity Force is a good choice because of it's AS, Critical Strike, AD and Movement Speed bonus. If you have a main tank in your team, go for Trinity Force.

- Bloodthirster is a very good item for Olaf. It gives you Attack Damage and Lifesteal, Lifesteal that fits perfectly with the one that you get from your Vicious Strikes. And let's not forget about that bonus 40 AD and 10% Lifesteal that you can get from this item's passive.

-Not very used on Olaf, but that doesn't mean that it's not a good item. It gives you some extra Movement Speed, which is great. Also the Attack Speed and Critical Strike are not to look down either, since Olaf's Attack Speed is not that high when he is high on health.

Defensive

-This is also a very good item for Olaf. It gives you a huge Life Regen, and some Movement Speed that will really help you. Buy this item if the team you are facing has some AP characters and you can't handle them. You can take advantage of the 76 Magic Resist and will help you defeat the AP champions.

-You're having trouble facing the enemy team's AD characters? But Thornmail. It will give you a HUGE advantage because of it's 100 Armor. And that passive is just....daamn ! It returns 30% of the damage taken as magic damage. You will crush your enemies. And it's just 2000 gold. Don't hesitate to buy this item if you need it.

-If the enemy team has both, AP and AD characters that gives you a tough time, just buy Guardian Angel. 68 Armor, 38 Magic Resist. You will be more durable in the game and it will be harder for the enemy team to take you out. Also it revives your champion with 750 HP and 375 Mana. And Olaf is very dangerous at only 750 HP. Huge attack speed, and if you activate your Vicious Strikes, you will be able to take out an enemy.

Olaf is a strong laner. He can solo lane easily, without any problem. You just have to play careful in early game, than you can become aggressive. If you are solo laning, use wards to keep your lane safe, especially if you play against champions like Twitch, Evelynn, Shaco and other stealth champions. Try harassing your opponent with Undertow, and when you have the chance, use Reckless Swing to hit them hard. If you get low on health, activate Vicious Strikes and try to hit as many minions as possible, to take advantage of the Lifesteal.

If you are playing solo top, and your opponent is also solo, you can be very dangerous. Place a ward in the river to prevent ganks, and than push your opponent, farm minions and don't let him do that, destroy his tower, kill him when you have the chance. If your opponent went back to base, you can take advantage of that with your Teleport. Gank other lane, go back to base, heal, get an item and than Teleport back to your lane. If you need help in your lane, tell your jungler to come and gank your enemy one or two times, so you can get some extra experience and gold, and after that you will dominate your opponent.

Ganking with Olaf is a MUST. You should keep an eye on other lanes, especially on those where your opponents play aggressively. Even if you won't kill the opponent that you are ganking, you will force him use at least one of his Summoner Spells, and after one or two levels, you can come back again to get the kill. If you want higher chances to get a kill from the first try, get the red buff before the gank. That buff will give you higher chances for the kill, because it slows the opponent that you're attacking.

Don't risk by turret diving, especially if your opponent has stun, because you can get killed before you have a chance to finish your enemy off. Turret dive only and only if you know that you will get the kill without dying.

In a teamfight, you play a very important role. Olaf is a tanky champion, so as soon as the teamfight begins, focus the most dangerous enemy. Activate Ragnarok, Ghost, and run for that enemy. DO NOT focus the tank. Focus the enemy that lands the most damage, and take him out. Because of your high health, you will take him out before their them gets you to half HP. Don't forget that you have your Ragnarok activated, so don't be afraid to be disabled, and don't forget to activate Tough It Out. This spell will help you shine in teamfights.
Olaf can easily 2v1 against almost any 2 characters from the game if they are not fed. Use your Ultimate and natural Lifesteal to take out the most squishy person first.