Hey I'm TomoSlayer, I'm Currently a top/jg Olaf main who plays on the oce servers.
I've played with many different Olaf builds (yes even crit and AP) I've finally settled down to the this build, runes, masteries and play style

Pros:
-Doesn't Need a lot of kills to be strong
-Reckless Swing deals true damage, good against squishes
Cons:
-Lack of Gap Closer
-Missed Undertow tend to be regrettable
-Bad CC

Farming (or cs'ing) with olaf is quite easy if you level Reckless swing first, at level 5 it deals 250 base true damage with a 40% ad scaling ratio this can be used effectively for mid range poking and grabs minions, this "cs tool" is very useful since it only costs health but its refunded if the swing kills the creep, Olaf's axes can be used aswell for creep farming on caster minions (better to use when pushing the lane, or when your opponent is missing) but aim your shots perfectly try hit at least 3 creeps at a time with it, if you use 1 for 1 creep it will be wasting mana.

Personally I like to max Reckless Swing first over Undertow just because of the true damage to tanky top laners or squishy laners, like Yasuo who can dodge your axes easily with Sweeping Blade and Wind Wall but because most laners start with low health the Reckless Swing is golden.
Speaking of Yasuo yas counters a Undertow first Olaf, but Yasuo does not counter a Reckless Swing first Olaf, Yasuo is the perfect reason why you should max Reckless Swing

Grab your ultimate Ragnarok whenever you can it offers 10/20/30 bonus armor and magic resist passively, but while active for 6 seconds you loose this bonus

Your w Tough It Out is what some people call a one point wonder, I personally don't call it a one point wonder when since it offers quite a bit to a team fight/solo lane since it has 40/50/60/70/80 % attack speed steroid and 9/12/15/18/21 % life steal steroid, this is big if your going full life steal/damage or even tank going in with that steroid and ultimate changes teamfights for your enemy they have to focus you down.
NOTE:
IF YOUR VERSING A RANGED OPPONENT MAX Undertow FIRST, IF YOUR VERSING A MELEE MAX Reckless Swing FIRST.

Lots of people question Ghost over Flash this is personal preference since olaf doesn't have a hard gap closer Flash can be taken but Ghost is so much better for chasing in combination with Ragnarok and Turbo Chemtank for a much needed catch up buff.

I start with Doran's Blade for the early game sustain (and it stacks with Tough It Out nicely) Health Potion's will stack with your Tough It Out again, and Stealth Ward to stop the brush garen's and future ganks

I normally rush a Spirit Visage for the tank stats and the increased healing.
Then I go Hexdrinker to build into a Maw of Malmortius later, it provides a nice stack with your passive Berserker Rage that grants you 25% attack speed bonus and 10% life steal when life line / life grip triggers

Since your most likely to be targeted down by all the ad carries Thornmail is an excellent choice for rushing straight into the back lines away from the tanks and kill the squishies.
Frozen Heart is useful when laning against attack speed champs like Jax and also when you run into the back lines in team fights.

Titanic Hydra is personal preference over the Ravenous Hydra because of your w Tough It Out giving you the life steal steroid it pretty much offers 2 Ravenous Hydra's in an ability which you get for free, why get an item that costs $3600 when you get the life steal stats for free, this makes the Titanic Hydra tank item much better in my eyes.

Turbo Chemtank oh boy I love this item on olaf (so does the god himself faker) its good for rushing into teamfights towards the back lines of the carries, its good for the health, provides a little bit extra mana and some more health regeneration, overall good item on olaf.

Blade of the Ruined King provides a little bit more life steal, percentage damage, a good attack speed steroid, and provides a mini slow, overall much needed item for a attack damage olaf.

Sterak's Gage since your in the backlines of the enemy being able to go rage at the enemy is fun enough, a shield to pull a big NOPE when the adc tries to hit you and increased damage to pretty much 3 shot the carries, this item is a massive deal for olaf.

The key masteries are as followed (everything else is preference based)
Fury for the extra attack speed.
Vampirism the extra life steal offers more sustain in lane.
Battering Blows armor pen is much needed if your against heavy tanks.
Fervor of Battle since your going to be auto attacking a lot these stacks are nice.
Assassin the extra damage when in a solo lane is a lot nicer than biscuits.
Merciless this is fun for dealing more damage.

Preseason edit: Okay most people are calling the game league of thunderlords, its not that case for some top layers/Junglers who are looking for on hit affects (like Jax) champs that have high poke should take thunderlords, but melee bruisers should take fev.

Your Reckless Swing is your main trade/poke for early game, it costs only health and gets refunded if it kills the creep/player. your Reckless Swing deals true damage to your opponents but reflects some of it to you.
when your laner (i'll use Lee Sin for example) goes to hit a creep smash a Reckless Swing if he backs away throw a Undertow to him to get a slow on him, then go for gold and auto attack him, with Reckless Swing's in the middle Lee Sin will be limited to using his ranged abilities to even get close to hit creeps.

I like to play neutral (trade when they trade) until lvl 3 when you have 3 of your much needed abilities, you can Undertow the enemy laner when they go in for a creep, you can Reckless Swing them when they go for a creep.
HUGE NOTE:
Don't use Undertow if the laner has a dash/good gap closer, they will avoid every one of your axes, this is why I level Reckless Swing first for the annoyance of trying to hurt a Shen who will dash every time you try to Undertow them


As Olaf your main goal in a team fight is to get into the backlines of the enemy and quickly kill the enemy squishies,
Note:
Olaf should be the fight engager for your team, if the enemy Blitzcrank tries to grab you just ult and engage to the fight from his mistake.

I cannot stress this enough ENGAGE WHEN OLAF ENGAGES, DO NOT WIMP OUT WHEN THE BESERKER JUMPS IN THE MIDDLE OF THE ENEMY

let Olaf take the crowd control effects from the enemy, Olaf is the charger to your phone, you need him to team fight.

TARGET, TARGET, TARGET

I need some feedback on my playstyle/build. I've already told a friend who is an Olaf main about the Reckless Swing first olaf and he loved it more than the Undertow first everyone does these days. he went from loosing 50% of his games to snowballing almost all of his games.
He said: "this changes counter picks to Olaf its that changing, playing around a massive mid melee ranged poke, instead of a skillshot axe shot, this makes olaf have many different play styles to the traditional undertow spamming."

This friend also plays with the Thunderlord's Decree mastery it makes for even more stronger poke potential with Reckless Swing (it also makes it look cool when 2 bolts of lightning hit at the same time)

Feedback is needed so this Guide can expand to show newer players the awesome powers behind the berserker.