This is my first guide on mobafire so I apologize in advance if there are any mistakes/things that don't quite make sense. Please read and try the guide before you vote on it as i find this way of playing Olaf to be largely reliable. Olaf is an extremely item dependent champion who excels at a lot of things. One of these is playing the role of the team jungler. This is because he has good ganks.

The reason behind the choice for these runes is that it gives Olaf the damage output he needs while also providing him with some defensive attributes early on that help when ganking by reducing the amount of damage you would take from say the enemy which you are ganking or the enemy turret if you decide to dive them.

These masteries help Olaf greatly when jungling as it increases many offensive stats as well as making you a little more beefy to take more damage.

I have recommended these items because Olaf is an off-tank melee dps champion. This is because Vicious Strikes grants attack damage for a short duration based on your max health. The thornmail and force of nature are so that you can take a substantial amount of harass and still be strong. The Warmog's Armor and Atma's Impaler are for more survivability and damage due to the Atma's Impaler Unique Passive which grants additional attack damage based on your max health.
The Mercury's Treads are because you are a melee (off-tank/dps) champion and therefore these boots are essential as they reduce the duration of crowd control placed on your champion by 35%.
The Elixir's are for more damage output while the oracle's is so that you can remove any wards that may have been placed around the map, any of Teemo's shrooms if the opposite team has one and so forth.