I play Jungle Olaf a ton and this is the most effective build I have found. Olaf is one of the most underrated junglers and can deal tons of damage in a short amount of time. Once you get to the point of having an Atma's Impaler with Frozen Mallet and Warmog's, you are UNSTOPPABLE! To add on top of that, why not get the Bloodthirster to bring your lifesteal up to +50% with your W activated.....

If you have the masteries and runes set up exactly like I have it, you will be able to clear you jungle at least twice. I start at blue having someone leash obviously and upgrade your W as much as possible after you have all your abilities. You need the lifesteal to keep your health up as you use your E and rampage through the jungle. Once you clear blue, get wolves, then head over to red. Once you have both red and blue, run through the jungle as many times as possible until your blue runs out. You shouldn't die anytime as long as you keep upgrading W all the way. Once you have Madred's or Wriggles, you can start ganking. Whenever you gank, make sure to use Ghost if you don't have boots or even if you do. You need to land your Q on the enemy to slow him, and make sure to pick up your axe after you have thrown it to reduce cooldown time so you can throw it again if needed. Have fun and be a good sport!

These items need to be built in this order to be the most effective build possible. If you need to be more of a tank rather than an AD, I recommend stacking Warmog's.