In a land where CC is king, rises a drunken axe wielding champion who will not be restrained: Olaf, the Berserker

Undertow has a strong AD ratio. It has CC. It's AoE. Skill it.
Tough It Out increases AD and thereby enhances Undertow.
Reckless Swing does nice damage when maxed, but requires close range. Max it last.

Note

Undertow scales on bonus AD, not total AD. The AD ratio is calculated by total minus base. Base AD is 54.1 +3.5 per level.

Attack Damage for reds works best since this is the fastest, easiest way to boost Undertow. Remember, Undertow scales on bonus attack damage.

Armor works best for yellows since it provides tankability.

Magic Resist for blues. Again, tankability.

Quints can be a number of things. I like movement speed runes, even though they are expensive. Attack Damage also makes sense. So does health.

AD is the primary focus because Undertow utilizes it and this is the primary attack in Dominion, whether it's used for harassing a tower, defending a tower, chasing down gank targets, or fleeing!

This build is intended to be a well rounded composition geared at taking down balanced opponents. Items synergize with Olaf's abilities and with each other. These items provide AD as well as a degree of tankiness. Unbalanced opponent teams should be exploited by incorporating Thornmail for AD heavy teams or Odyn's Veil for AP heavy teams.

Boots

Mercury's Treads - not as crucial as usual, since Olaf's ult removes and prevents all CC for 6 seconds
are reasonable choices

Finer Points

Most players should be familiar with Prospector's Blade. It is one of the best opening weapons because of the price to benefit ratio. I.e. It's cheap and really good for what you pay for it.

Frozen Mallet on the other hand is not cheap. But it's too amazing to overlook. 40% slow on every hit! Lots of health (which boosts Tough It Out)!

The Brutalizer and then Youmuu's Ghostblade is really quite effective. Cooldown Reduction and Attack Damage are very desirable on this build. The other stats are quite nice for melee fights.

Now we get to Tiamat. This is more than a splash effect. It's 50 Attack Damage for about 2k gold. This is, by far, the best Attack Damage to price ratio in the game.

At level 18, Atma's Impaler will give Olaf an additional 60 Attack Damage. Nice!

Best of all, you should be able to build all the items before the end of most games. Typically, you'll be done building before anyone else due to the price concious nature of the item selection. If you are doing really well and have lots of juicy kills, you might even be able to afford selling the Prospector's Blade and buying a Spirit Visage or even a Sanguine Blade.

The priorty from most important to least important goes Ragnarok, Undertow, Tough It Out, Reckless Swing. The reason for this is leveraged largely on a particular suggested play style and skill rotation.

The primary source of damage comes from Undertow and is used in conjunction with either kiting traditional melee opponents or locking down and melee axing squishy ranged opponents.

The rotation is to activate Tough It Out then Undertow. Tough It Out amps attack damage and allows for devistating hits with Undertow. Reckless Swing gives additional damage once the target has been slowed by Undertow and Olaf moves in for the kill. Finally, Ragnarok provides an escape if things turn ugly, or an extra edge in a close fight.

Your main goal with this build is to immobilize enemies with Undertow so that you can your team can wail on them. When possible, be sure to pick up the axe after throwing it so you can spam it over and over.

In the absence of a more suitable tank, you will tank towers when capping with teammates. Prior to capping, you should harass with Undertow.

When defending, throw the axe through minions and as many champs as possible to get their HP low before they move in to your tower.

Pros

Brolaf has a ****ing beer dispensing helmet
Solid CC
Excellent DPS
CC immunity with the ult

Cons

Dependent on landing skillshots with the axe throw

Olaf is more powerful than most people realize, especially since most other builds focus on maxing Reckless Swing right away. Undertow is Olaf's real shining skill and Olaf players should not hesitate in exploiting it.