Sup Bros. I have mained Olaf for a long time now as he was the first Champion I ever baught. Olaf is an awesome champion and is usually viable with any team, his ability to off tank and destroy the other teams carry is amazing. Depending on your team comp and the other team, you can build Olaf accordingly. Usually I prefer to build him as aan Anti-Carry. (On a side note this is my first guide so any tips and helpful commentary would be much apreciated.)

The runes I take are pretty standard, Armor Pen Marks and Quints, Armor Seals, and Magic Per Level Glyphs.

For Masteries I go 1/8/21, this combo allows Olaf to clear the Jungle fast and efficently as well as move around and gank fast. Improved Ghost and Quickness in the Utility tree are the most important, the bonus movement speed+Olaf's Ulty during teamfights= a dead enemy carry.

Why the 5 pots and a Cloth over the Vampiric Scepter? The pots and cloth allow Olaf to clear the jungle completely as well as throw out a gank or two if your really feeling Salty. Olafs core build is. Frozen Mallet, Wriggles, and Boots. Your boots like all games is completely situational. However for my build as the Anit Carry I get the boots of Swiftness because the movement speed once again helps you get to the squishys faster.
Frozen Mallet is Olaf's sexy time. By far the best item you can get for him the health and slow are awesome.
The final three items I put on the build are just usually what I get for a standard Anti-Carry.
I didn't add a Ghostblade but it is super awesome on Olaf as well. The Starks armor pen just adds to the devistation and the Atmas+Warmog just makes him even tankier.

Undertow vs Reckless Swing?

I maximaize Undertow first because it is Olaf's greatest early game ability. It allows you to clear the jungle faster as well as do more damage with your ganks. Yea leveling your Reckless Swing can cause more damage but being able to continousuly pick up your axe and throw it and keep doing damage and slowing them is much more important early game.
In between leveling your Ultimate, level Undertow,Reckless Swing, and then Vicious Strikes. Vicious Strikes is just not what it used to be 1 rank of it should be enough for you to clear the jungle.

Pretty self explanatory, Ghost+Mastery Tree movement speed+ Ragnarok=Dead Carry. Yea i've gotten pretty redundant. But get over it bro.
Why smite? If your not taking smite then your not jungling end of story.

Alright so if you actually read all of this then you get a freakin gold star for the day and an awesome car from my dads dealership bro. But yea Olaf is an amazing Hero, you can carry super hard with him and people fail to realize how valuable he is. My final words are if your Anti-Carrying, wait for the tank to go in and try and be sneaky, lead with your Q. If your Off-Tanking don't be afraid to initiate and set up a Malphite ulty or an Aumumu bandage toss.

Going to steel a jungle video from stonewall, but you gota admit his videos are pretty helpfull. (Note in the Video Stonewall is using 0/21/9)
http://www.youtube.com/watch?v=JdlwLoTKRAY

Olaf is great in team fights. You can hit the TANK with your wreckless swing since its True Damage, or hit the Carry or anyone really. The biggest thing with Olaf is determining what your role is for the team before the game starts. Depending on if your peeling and protecting your carry or just going straight for theirs.