Friends olaf build to help him remember what he does, and what he needs to get, feel free to use it

olaf with armor pen is amazing, his ult gives him a lot, and with runes you can get 25, with masteries you can get an extra 6 totaling 31 armor pen beginning game

Ghost and Exhaust are probably the best summoners on olaf but you can mix and match if you'd like, ghost gives you the extra speed boost to make you catch up to people in case they flash over a wall, you can throw your axe and slow them and then just run to them, with frozen mallet doing a 40% slow makes you be able to catch up to anybody almost imediatly

you can build olaf in a lot of different ways, i dont necessarily like this order but this is my friends order, i would personally get brutalizer then build right onto phantom dancer and black cleaver then finish off with you'moos

olaf is a nice last hitter, my friend gets the creep killing mastery that does an extra 4 damage to creeps but that is really only good if your jungling olaf, which in my opinion isnt very good but isnt very bad either, just simply average