This is my first build guide. I know it may not be the best, please go easy on me.

I have played olaf for quite some time now and thought that this build has really worked well. I really do not like squishy, pure AD champions but still like having a lot of damage. This is why olaf was perfect for me. He deals a lot of damage without giving up that extra tankiness used to stay alive.

Greater Mark of Strength: This is really good on olaf because early game with a ruby crystal you really smash. This mixed with his passive is really powerful.
Greater Glyph of Shielding: This is really great for defense because each time you level up you gain 1.35 magic resist adding up to +24 at level 18. Yes this doesn't seem like much but it will make a big difference against mages.
Greater seal of Resilience: Defense, defense and defense! With 9 of these that adds up to a total of 12.69 armor at level 1! This is mostly for early game survival. After you get Wriggles and Atmas your armor will just build up a lot without needing something like thornmail
Greater Quintessence of Strength: Once again, lots of damage early level = Pwnlaf.

Flash: Flash is both a perfect get away and a perfect chasing tool. Why not ghost? When running, olaf has his ult that can break him out of stuns, slows, silences, etc. so flash really isn't needed there. When chasing, if they are far away all you need to do is flash and land an undertow to slow them.
Ignite: Well, perfect for that extra damage in a 1v1. Also very good when an enemy is fleeing.

Ruby Crystal: Very good for early game, that extra health lets you take more damage when fighting an enemy early on. I Get it instead of dorans shield because it gives more health and it means you are closer to warmogs.
Regrowth Pendant: Very good for sustainability this lets you minion farm while healing meaning that you can stay in the lane for longer. 1 step closer to warmogs.
Giants Belt: You should try to rush for this as fast as you can, if you are finding it hard to get minion kills then try to go in the jungle a bit. If you get this early enough it can really benefit you and you will be almost at warmogs.
Berserkers Greaves: Gives you the attack speed you need, mixed with his passive you will be really strong.
Warmog's Armor: One of the best items on olaf without doubt. With his passive even at highish amount of hp you will get a lot of extra attack speed.
Wriggle's Lantern: This will give you a bit of extra attack damage and armor but the main reason for getting this is for the life steal. This along with viscous strikes will give you a lot of life steal and is very good when you need some health.
Atma's Impaler: This is good because it gives you some good armor, a little bit of critical chance but most importantly the passive; You gain attack damage equal to 1.5% of your health. With 3K health (which you will get with warmogs and frozen mallet) you get 45 attack damage. This means you pretty much got a free BF sword with the addition of armor and critical chance.
Maw of Malmortius: Why? Magic resist, only a little less damage then a bf sword, gives you a spell shield if you are about to die from Magic damage and adds 1 attack damage for every 2.5% of health missing. Works perfectly with your passive.

HEAVY AP
Force of Nature: If you're against heavy casters / a lot of AP burst damage this is a must have for you're final item.

HEAVY AD
Frozen Heart: This is a excellent item against heave AD enemies, but why not thorn mail? I feel that at the late game stage frozen heart is a much better choice then thorn mail simply because of it's passive (Reduces enemies attack speed by 20% within a 1000 range). This means if you are up against heavy DPS as well as heavy AD you will survive much better.
Phantom Dancer: If you feel that you are attacking too slow then you must get this. (Take armor / magic resist over this if needed). Phantom dancer adds a lot of DPS and mixed with olaf's passive and his berserker greaves he will be attacking really fast while being able to survive for a long time.
Frozen Mallet: Need a little more health? Then go ahead and buy this. I don't see the need for another warmogs tbh. Getting a frozen mallet increases your damage and slows your opponents. This is really good in team fights if someone tries to run you can catch them off.

FARMING
Farming is very important early game. If you don't get warmogs fast enough then you will be really behind. Try to last hit minions. Sit back and let your minions do the work, when the enemy minions are low go in for the hit.

CHAMPION KILLS

While waiting try to harrass the enemy with your undertow. When you reach level 2 you can start getting a bit more aggressive. Reckless swing does a lot of damage at low levels. If you upgrade it enough at low levels you should be able to half someones health with the help of your team mate. If you go to reckless swing them and they start to attack you, don't worry you should be fine. Use your Undertow for some extra damage and your vicious strike to grab some life steal and damage. If they start to run undertow them again, flash if you need and then try to land a reckless swing. If they are low enough feel free to ignite them and run off.

If you get into a fight and things aren't going your way chuck an undertow at them and run. If they try to slow or stun you then ult and keep running. If you need to you can flash.

You want to lane with someone who has good CC (crowd control) or AOE (area of effect). Try not to lane with someone who is a carry because it will make it much harder for you to get gold. The reason you want someone with CC is because they can slow them and you are able to land attacks in without taking much damage. AOE is also good because when they attack champions or minions it will splash onto enemy's around them. A good example of this is Kayle.

I play with 2 of my friends so we play twisted tree line. We have a Kayle, Olaf, Kogmaw combination and barley ever lose a game. Kayle and I go bot and Kog goes top. I use this build, and as I said laning olaf with Kayle is VERY strong.

Early game

You want to try and farm alot, last hitting and getting as much gold as you can. Try to stay in the lane as long as you can, buy health pots if needed. Try to harrass the other team and if you see a chance to get a kill take it.

Mid game

By this point you're carry should be really strong and you should have a lot of gold. If you need more gold then just stay in your lane farming. If you see a gank / easy kill then go ahead. Olaf is very good at getting away from enemy's so don't worry about that.

Late game

By now team fights should be happening and towers should be pushed. You should be the one initiating the team fights. This is good because you have a lot of health but also a heap of damage meaning you will be able to tank out damage while getting people low for your team to finish them off.

Thanks for reading my guide. I hope you use this build and enjoy Olaf as much as I do. Please comment and remember this is my first guide so try to stay away from the mean comments. :)