Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Right, Olaf is basically a character that gets stronger as his HP becomes lower.
Life Steal is very important for Olaf because as his life becomes low, his attack speed increases and makes him heal more as he hits, probably putting the damage he's taking and ammount he's healing in equilibrium so he can always survive most battles even if he's about to die.
Level 1-6: During this Period, we should focus on the boots. We need Olaf able to run fast so getting Berserk Boots first is a good thing. At this point you should play defensively unless you know you can get the kill. Do not harass much this early in the game because you'll need to save your MP. You can either get Phage or Zeal first, it doesn't matter about the order but the thing you should not get is any more potions because they'll be a waste of money. Save up money and try last hit Minions, every little cash counts.
Level 7-13: You should now have both Phage and Zeal by now and either have or saving up for The Brutalizer. You may run around the Map to pick on Enemy Champs who have close to 1/2 HP or maybe even lower. Remember to aim long with the Q Skill in order to slow them for a good chance of a Kill. During these levels you should be able to complete the Froze Mallet and if you're skillful enough, probably a Bloodthirster and half way to a Phantom Dancer.
Level 14-18: To be honest, if you're playing it right, you should be practically unbeatable, but do not think of yourself as a Tank. You should probably organise some Ganks. Participate in more team fights and victory shouldn't be far away. (But this also requires your team to play good aswell.)
Olaf is a risk taking Champ, and taking risks may bring some good things back in return.