Hi, I main Olaf, have since he came out over a year ago. I've played countless games with him, bought every skin, played every build type(Dps, Tank, Tanky dps, crit, Attack speed, Lifesteal, etc), some very bad, some very good, but personally from experience, the build I'm showing in this game is the best build I've used. After looking at TheOddOne's guide, which is very good btw, I tried it and found it to be excellent, but of course I felt I had to adjust it a little bit for myself and try and improve it.

I roll Armor pen marks, Armor pen quints, Mag resist glyphs, and Armor seals.
Armor pen is just plain fantastic for most junglers and tanky dps, especially with Olaf as it synergizes well with his ultimate.

Armor and mag resist runes are used for early game durability, You end up going for a warmogs, and having the resist runes help post pone the need for any armor or resistances.

These are pretty basic runes, rolled by most junglers/olafs.

Utility masteries are perfect for most junglers. The extra movement speed, lower cooldowns, extra mana, is extremely helpful, add in some armor and mag resist and you have yourself the best jungle masteries for Olaf. These are pretty much the best masteries for Olaf, everyone who's anyone probably uses these.

The basic jungler item.
Boots are personal preference. MIne are greaves because after 30 minutes, you are unstoppable anyways, and after playing for over a year, my micro with Brolaf is high, with Ghost+ ult i do not need Merc treads, and if i dont have ult, i dont get myself into bad cc positions. Preferential, like i said. Ninja tabi might be good vs heavy as/ad teams like Yi/xin.
Warmogs, the bread and butter of awesomeness on my boy. with this item, your 1 step away from being nearly unstoppable.
With this, you'll have 180 AD, 3k health, plenty of attack speed, plenty of armor. the warmogs+atma's combo on this guy is so strong, it synergizes so well, its insane.
Starks for the attack speed and lifesteal, a good buff for your team as well. at this point you really dont need more attack damage so bloodthirsters is lackluster, but you still need lifesteal for sustain, this synergizes well with Olaf

Force of nature if they roll magic resistance, frozen heart if they roll heavy AD, and the last item is situational.

Start at blue, smite when u use vicious strikes. run through wolfs, wraiths, lizard, golems, then go B, then gank. when you B, buy boots to gank better.

They say Olaf needs buffs, that he isnt as good as other junglers like udyr, and nocturne, i disagree, i think if played to his potential, he becomes the best jungler. Because after 30 minutes, Olaf is unstoppable. Not only are you basically a tank for your team, your sustain with lifesteal/hregen, is absolutely humongous, almost always leaving you at full health after they use their burst when you attack a few minions. Once you finish your atma's and get into a starks, its basically GG unless your teamates are complete ***. good luck