Not Updated For Current Season
First place I wanted to thank moiseman for his build "Maximize dmg & survivability", this build is pretty much the same as his, with some modifications i made to improve survivability at early game, since I prefer to be aggressive on enemies from lvl 1 and to do that you're gonna need a lot of hp's. HP runes and ruby crystal will make Olaf a meat shield at the start, giving him something around 900 hp's at level 1, so you don't need to worry about open fights.
I don't think i must explain exhaust and ignite, but if you're new and don't understand them, here's the tip: when they start hitting you and don't see your hp bar going down, they'll flee, that's when you use exhaust, so you can keep hitting him, if he doesn't die after exhaust, his hp bar will probably be low, that's when you cast ignite and watch them die by their turret. If you got a partner with stun or slow, you guys will probably get the first blood in the game before 30 secs if enemy comes in the bushes.
Now the items. I decided to pick ruby crystal and giant's belt before boots because it'll allow you to start focusing on killing players earlier in the game, or if they're too defensive, you can at least farm minions on your lane and don't need to worry about their harrassing. After you get those 2, you should get some boots, I always go for swiftness, because when you get frozen mallet everyone will run from you on sight, so you need speed to get close enough to hit them, but it's a personnal choice, you can pick other boots if you got allies with cc's. After boots, get frozen mallet and you're pretty much a killing machine, now it's time to start ganking lanes and pushing towers. Try to get some kills to earn enough gold for warmog's. If you got warmog's, the game is probably coming to end, but if it isn't and both teams are doing good by this point, it's time to evaluate your enemy, if there are 3 or more casters in heir team, your next item should be force of nature, if not, go for randuin's omen. Of course, if there are only 2 casters in the enemy team and they're owning your team, go for force of nature. Make sure to pick the best choice to help your team.
To finish this explanation, I wanted to present Olaf like I see him. He's a great tank/dps and I think you're gonna help your team the most if you go for the hp build. To be honest, I've never tried the dps build, I don't know how it works, but after seeing what Olaf can do in the battlefield with this hp build I don't need to try any other build. You can easily hold a lane by yourself if you got a jungle WW, Fiddle or Udyr in your team, after reaching lvl 6, you won't have troubles to get kills on your lane even if it's 2 on 1, target the weaker enemy, cast ragnarok and go for the kill, no need to worry, he will die before they can take you down. You'll lead your team in turret pushing and team battles, since you'll probably survive most teams and get a lot of kills with reckless swing. My best rate with him was 16/2 in a game with a 0/12 sion and a 4/10 akali on my team and the game i killed most e's with him was 21/5. If you're looking for a solid champion for the entire game, get Olaf with hp build.