FIRST BUILD DO NOT FLAME PLEASE!
I choose Exhaust because if you do not hit them with frozen mallet at first it is good to slow them down with a spell and then kill them with all your viking might.
I also chose ignite incase at the beginging you use your viscous strike and you parter uses a spell/ability (if they try and run you can use ignite or exhaust to kill them), and kill them using ignite.
You should start out with Regrowth Pendant so you can stay and kill minions/Champs.
When you have enough for Mercury Treads (for the magic resistent) I would go and get them, Then come back with High enough speed to catch them with exhaust.
You can get a phage (for the frozen mallet) But if you don't have enough yet you can get it hopefully mid game.
You should have your phage and on your way to slow down some champs/gank mid when needed.
Kill some champs and you should now have enough for giants belt/Frozen mallet after that go farm/gank tons of champs in mid, top, or bottom lanes.
By now you should have enough for you Warmogs armor or just enough to get another giants belt.
Late game you should have your force of nature. After that your good to go for some chasing with your exhaust and frozen mallet.
Save up for thornmail and those dps champs will be crawling away in misery :).
Better hurry and save up for that guardian angel because you will need it too kill some turrets!
Force of nature
Frozen Mallet is good for slowing down people since olaf is not very fast at all. and it supports your team incase people are trying to gank and you are there too slow them down.
Warmogs armor is used for the Hp, and the Hp regeneration.
Force of nature is for the speed, Magic resistance (casters like ryze), and the Hp regeneration.
Thornmaill is of course for the Dpsers. That little 30% of magic damage can go a long way if its a teamfight.
Guardian angel is used to kill the other team if one of theirs is low Hp. And to get away if your very low Hp when you come alive again.
Now its time for the why questions.
Usually people do not think to use thornmail on olaf, I do because it will give you the power you need to kill 2 people or even 3 in a teamfight with another champ, Or even yourself alone.
Everyone usually does a tank/dps build but Im doing something different with that thornmail there :).
Why those Runes?
The runes will be for more attack speed/critical chance.
Why did you choose the items you chose?
To be different from other olaf tank/damage builds :).
And because this build works, in many different ways.
Well I hope you like this build :).
Again this is my first one please do not flame.