This is my first build, my jungle 3v3 olaf. Have had lots of success in games with it. Not the best lol player ever, but I've probably tallied about 2000+ 3v3 wins, so I think I know what I'm doing somewhat. Skip to the summary at the end for an easier explanation of the build. thanks!
. Essential for the early ganking. Armor/MagRes for added tankiness (armor needed for jungling). AD Reds give you some added dmg, as well as added dmg on your undertow (taken at lvl 1).
10-11-9 gives you some good balance. 10 offensive masteries for dmg, attack speed, and armor pen. 11 in defense for some added tankiness, tough skin and bladed armor for jungling, and a point in summoners resolve for the added Smite gold (adds up if you smite often). 9 in utility for the added movespeed, increase in buff time, and a point to make your flash cooldown a little lower.
: Enables for really fast jungling. Using smite after activating W gives you a decent chunk of health to help jungling with as well. Gives you good lizard/dragon control. Also will heal you slightly when used while W active.
: Classic ganking and chasing spell, has a plethora of uses that I'm sure I don't need to explain.
Creeping / Jungling
Olaf can obliterate the jungle, just beware of counter jungling, as he will usually be around half health or lower a lot of the time to jungle efficiently (before wriggles). Grey/Green buffs can be taken at level 2-3 with around full health to start it, and smite. Use all skills, smite while W active to finish it off. Don't bother with other two creeps, as they will usually kill you if you stick around (or a counter jungler might find you). Grey buff can be utilized best because you can attack and smite it, run away from other ghosts, recall, shop, and be back in the jungle really fast because of the buff (mastery also lets you keep buff longer).
Red buff can easily be gotten the same way, smite may not even be needed (help is always welcome from bot lane). Olaf with red + undertow + all the movespeed will make you a ganking force.
I wouldn't risk starting dragon until wriggles, though it's possible with just madred's if you are lucky (or your friends help you. After wriggles everything is pretty easy, just E everything as much as possible.
Start with Boots and pots, this will start you out with 399 movespeed with undertow Q and level 1. Don't need cloth armor to start because Olaf's atk speed will rise significantly when at low health, using potions is all that's needed.
This build is highly dependent on early ganks. With undertow, can outrun pretty much any champ that doesn't also have boots + quints. Flash over walls for easier ganks as well. With smite, can get green/grey/red buffs by lvl 3 easily (careful at grey buff, just kill the buff ghost and run from the other two).
Be sure to use your skills AFTER using W, for the small amount of spell vamp. THIS ALSO INCLUDES SMITE! W to Smite will give you a small heal.