This is my Jungle Olaf guide, it's built as an offtank, you're gonna have alot of health but also pump out some great damage.

Pros
-Ult allows you to chase carries by making you immune to cc (though if you're stunned you can't break it) also it gives damage reduction as well as passive armor pen, both good as well.
-Undertow's skill shot can go through targets making it do alot of damage in a team fight, as well as it's great for ganking because of the slow.
-Vicious Strikes allows you to jungle easily and gives you more damage by having higher health, it has great synergy with Warmog's and Atma's.
-Reckless Strike is great for anyone, but especially tanks, since it's true damage they can have all the resist/armor in the world and you'll hit just as hard (note if you don't have the health that you'll take as damage you won't be able to cast it)
-Your passive allows you to jungle very fast, staying near low health gives you 1.2-3 attack speed at level 1
Cons
-Though you can't be cc'd while ulting you probably will be focused right after if they're still alive and you won't have an escape mechanism.
-Undertow can be hard to land, and it's vital to throw it where you can pick it up making it tough to get use to using it for ganks.
-Though you'll have alot of health and with your ult up you may take reduced damage, you are NOT a tank, you are NOT incincible.

Red
Armor Pen, these affect almost all your damage minus Reckless Strike, great early, mid, and late game.
Yellow
Armor, this greatly helps you jungle, it allows you to stay at low health without dying during your first few runs through.
Blue
Attack Speed
I chose these because they work well with your Vicious Strikes and passive, though these can be changed to Magic Resist if you would rather that.
Quints
Armor Pen, again, affects alot of your damage as well as being great throughout the game.

This set of masteries gives you great utility, improved ghost along with some extra gold and a lower cd on smite, the 3% move speed and alot of cd reduction, as well as the armor/mresist and strength of spirit allow you (again) to jungle more efficiently

Basic jungler set up at the start choosing a cloth armor and 5 health pots.
After that you want to build your Wriggle's asap, that ward is going to help considerably early game, and it's cheap as well as giving some nice life steal and magic damage proc.
After that start getting Boots of Swiftness, the reason why I get these is because the other 2 options are Mercury Treads and Berserker's Greaves, I wouldn't get Mercs because your ult nulls CC and if you get CC'd after that you're probably sol anyways. And I wouldn't get the Greaves because you already get alot from your passive and Yomuu's, that extra run speed will get you more kills than the 25% attack speed.
Next is Yomuu's, which at first I wasn't a fan of, but since trying it, it's great. Arp, crit, cd reduction, along with a great use, it allows you to chase, and burst as well as pop this with your Vicious Strikes and annihilate a tower or structure. And combined with your slow and a Frozen Mallet, which is your next peice, there's no getting away from you. Especially with that hugh chunk of health you got from the Mallet. Next comes Warmog's, this item will further increase your health, but that's not all, since your Vicious Strikes is based of max health that effect will also increase, you'll want to stack this asap because thennnnnnnnnnnnnn.
Atma's! This item has amazing synergy with Warmog's and Frozen Mallet, and it gives some crit and armor.

Not gonna explain the skills in depth, they're pretty simple to understand, but the reasoning behind the order is because of the slow undertow provides, the greater the slow, the easier it is to gank people, after that follow up with Reckless Strike because it's alot better mid/early game than Vicious Strikes which scales alot better late game when you're building your Mallet and Warmog's. That being said, the passive, duration increase and increase in damage reductions means you should level Ragnarok first and foremost at 6, 11, and 16.

Smite
You're jungling this one is a must even if you think you don't need it, you do.
Ghost
I prefer this more than any other because along with Ragnarok it allows you to easily escape ganks or chase low heath champions running away, also helps with ganking, has alot of uses and is my facvorite on Olaf.

Another option to choose is flash, it serves the same purpose as Ghost but I find Ghost alot more effective, your choice though.

I could tell you what path you should jungle to, but instead I'll just show you this.
Play Video
This is credited to Stonewall008.

Though if you have someone willing to leash blue, you can start there and instead go Wolves->Wraiths->Double Golems instead of going Double Golems->Wraiths->Wolves, otherwise it's the same route.

ANYWAYS THAT'S IT, KTHX, comment to vote etc etc.