Pros:
-Very Tanky at late game
-Massive damage with E and burst
-Good chases
-Good laning phase
-Good pick for Elo Hell
-True damage
-Ultimate that removes any CC and gives you 100% Tenacity while active
-Ultimate and E Costs no mana.

Cons:
-Not naturally tanky
-Drinks mana early game
-Turns red with ult on
-True damage costs HP
-Very Item Dependent

Passive - Berserker Rage:
For every 1% of HP missing Olaf has bonus 1% attack speed

Q - Undertow:
Olaf throws an axe that damages and slows enemies it passes through. If Olaf picks the axe, The cooldown is reduced by 4.5 Secs.
PHYSICAL DAMAGE: 70 / 115 / 160 / 205 / 250 (+100% bonus AD)
SLOW: 35 / 40 / 45 / 50 / 55%
RANGE: 400 - 1000
COOLDOWN: 8
COST: 60 MANA
This is Olaf's Main slowing utility, But i prefer to max it last because your main damage is your E.

W - Vicious Strikes:
For 6 seconds, Olaf gains bonus lifesteal and bonus attack speed. During this time, Olaf also gains 1% enhanced healing from all sources for every 2.5% of his missing health.

ATTACK SPEED: 40 / 50 / 60 / 70 / 80 %
LIFE STEAL: 9 / 12 / 15 / 18 / 21 %
COOLDOWN: 16
COST: 30 MANA

It basically gives you attack speed, lifesteal and bonus healing. this is your 2nd main damage and this is why i max it 2nd.

E - Reckless Swing:
ACTIVE: Olaf attacks with such force that it deals true damage to his target. The ability's cost is equal to 30% of the damage dealt and the cost is refunded if Reckless Swing kills the target.
PASSIVE: Olaf's basic attacks will reduce the cooldown of Reckless Swing by 1 second.

TRUE DAMAGE: 70 / 115 / 160 / 205 / 250 (+40% AD)
COOLDOWN: 12 / 11 / 10 / 9 / 8
COST: 21 / 34.5 / 48 / 61.5 / 75 (+12% AD) HEALTH

This is your main damage. It is dealing true damage, ignoring all armor. THIS is why i max it first.

Ultimate - Ragnarok:
PASSIVE: Olaf gains bonus armor and magic resistance.

ACTIVE: Olaf instantly removes all crowd control effects from himself and becomes immune to them for 6 seconds. During this time, he loses the passive bonus but gains bonus attack damage.

ARMOR / MAGIC RESIST: 10 / 20 / 30
ATTACK DAMAGE: 40 / 60 / 80
COOLDOWN: 120 / 100 / 80

It basically acts like cleanse and gives you 100$ Tenacity and AD for 6 Seconds.

HP: 441 (+93 PER LEVEL)
ATTACK DAMAGE: 54.1 (+3.5 PER LEVEL)
HEALTH REGEN: 7.0 (+0.9 PER LEVEL)
ATTACK SPEED: 0.694 (+2.7% PER LEVEL)
MANA: 235 (+45 PER LEVEL)
ARMOR: 17 (+3 PER LEVEL)
MANA REGENERATION: 6.5 (+0.575 PER LEVEL)
MAGIC RESIST: 30 (+1.25 PER LEVEL)
RANGE: 125 (MELEE RANGE)
MOVEMENT SPEED: 350

RECOMMENDED:

Why ghost ignite? Olaf Has really good chases and escapes with his Q and once he gets his Bilgewater Cutlass.
Ghost ignite is very good on Olaf.

Another spells you can take instead of ghost (Not recommended):