I jungle with olaf because his passive makes it so easy and he is in my opnion one of the best gankers. If u are laning with him he is easy to kill, because he doesnt have the abilitys to win against 2vs2 in early game. the enemy can easy escape dodge the attacks of olaf. They can run away or something like that.
I start with life steal because his second ability got a cd of 12 seconds. If you dont steal any life when your second ability is on cd, you will lose against blue and fail with jungling.

I use some armor in my rune build, because you will take less damage of the blue monster. You can buy life steal as first item instead of cloth armor.
I take some attack damage to in my rune build. Because you will get more life back when you attack.
I use some health runes too, because you cant survive longer the attacks of the creeps.
At last. I use cd reduction too in my rune build. Because of simple reason. So you can spray faster with your abilities. For example in early game. you can spray with your thirth ability. You will get fast kills in early game, because the thirth ability of olaf gots that huge damage in early game.

My item build is based on my second ability. In teamfights you will be undestructable. Because if you active your second ability you got lifesteal, spel vamp and more damage -> based on your health. In my item build I already got some life steal, damage and health. I will steal many life of other players so I cant be killed.

One last thing. I use phatnom dancer because of the movement speed and the attack speed. But if the enemy team got many ability power. I sugest to take force of nature instead of phantom dancer. You will be more tanky,you will stand longer in teamfights alive, and still you got that huge damage and some movement speed