Olaf, there is many ways to build this champion, many ways to play with him, but in this guide I'll tell you how to build Olaf to destroy any enemy tank and survive to get away or continue killing with your team. In this build you act like your team's tank, but also the one who kills the enemy tank, so it's easier for your team to take out the casters and rest of their dps. Best way to start the game is Jungle. :P

In this build, you go in tanking masteries, simply because it gives you a high chance to survive from enemy damage.

Go for any runes you find helpful, I would choose armor penetration, health, dodge and attack speed.

Probably the best summoner spells would be exhaust and ignite, exhaust for disabling enemy burst damage and ignite to stop tank being healed, so it's easier to kill. You can also try flash/ghost and heal if you find those better. Smite is just a waste, Olaf can jungle fast even with out it.

Start at Jungle, with the two golems, following wolves and then red and blue buff. Base if needed, starting items should be cloth armor and as many health potions as possible. You should try stay in jungle until you have maxed you Q and W. Go help with team fights if they have begun before that.

Pros
+ High chance to survive, because of the summoner spells, ulti and items
+ Enough damage to take out enemy tank and survive alive
+ You can win every 1 on 1 fight
+ Fast leveling in the jungle
+ Perfect champion to help out in ganks in early games!

Cons
- You are not as tank as tanks should be, you are missing some armor and magic resistance
- Not enough health for proper tanking
- Slow movement speed
- You usually need another tank with you among the dps to make effort for the fight
- The damage isn't very high in this build