Well here is my first build and it's for Olaf the defender of the bro-code
i have tried a couple of builds and i have found them all to be great BUT often the items are very expensive and doesn't give you a chance to finish your build before you either loose or win im not saying this build is any better but atleast i've seen the problem and trying to solve it

Constructive criticism is ofcourse allowed ;)

i have found tiamat to be a great farimg tool for a champ that doesn't really glow in Multi-targeting plus it gives you a nice boost for ganks and Team fights early game cause of the 50 ad and high health regen and mana regen
When you wanna clear out a lane you just pop "W" and let the tiamat do the work ;)
makes creeping VERY easy

When / if you get to finish the build you sell Tiamat and buy either Atmas impaler or Infinity edge
Or if you having trouble staying up you drop buying zeal and go for either an AP counter item or AD counter item

Dont be a fat donkey do sell your dorans shield mid game to get the extra gold and room for the better items!

i usually pick up undertow at lvl 1 and harass as much as possible
(but if you team is going for first blood gank reckless swing would be an awsome replacement)
Then you focus reckless swing and ofcourse you pick up your ult as prio whenever it's avaible
it can be an idea to put two or three points in Vicious Strikes as soon as you get tiamat and then go back to focusing reckless swing i lvl undertow as the last skill since you just need the slow from that

well if your having new to olaf or to the game i would recommend going 21 points in defense since that will help you stay longer in fights get the extra lvling experince and olaf experince ;) and it can save ur but against some 1v1 / 1v2 situations

else i go for 21/9/0 for the armor pen and crit and damage increase it gives
but you should spec the way you think suits you

Pros
Can trick foes to think your weak when your 20-50 % health but you not ;)
i find him to be a good harras early game
you can solo most champs
you can be the turning tide in a team fight cause of your constant high damage output
great chaser

Cons
if you are denied early game you will struggle to get back in(harras but with style dont go in axe and blazin' all the time :> )
watch your health it's your best friend and your worst enemy
can easily be pushed out of xp range by a decent harras ranged
dont overestimate your powers make sure it's the other way around and make them underestimate your powers