Olaf is close range fighter.
Well played can be lethal.With your passive you gain a lot of atack speed, this can grant your win at the firstblood.
His strongest point is ignore CCs, this allowed him to kill the target without any one slowing,snaring or supressing you.

The marks and the quintessences should be of armor pen. this will greatly increase your auto atack and your undertow.
Using the evasion seals will alowed you to jungle easly and sometimes help you on the fights.
I recomend you magic resist glyphs this will make more tough vs casters at firstblood.

21/9/0
At the offensive tree the main masteries are sorcery, alacrity, sunder and havoc.
And at the defensive tree use the hardiness, evasion and nimbleness to combine with the dodoge runes.

I recomend you start with doran´s shield to 3vs3 and 5vs5.
I build olaf with some armor pen, heath, atackpeed and atack demage.
Get starks fervor because improve your stats and the stats of your mates around you.
Than buy Frozen malet to slow the enemies and become a little mroe tough.
You can decide what item should you buy now, because you already got the important items Beserker greaves, starks fervor and frozen malet.

You should learn all your abilities but start with undertow because you can check the bushes and slow the enemies with this skill.
Try to use your UnderTow in the tricky way, throw the axe to the unexpected.
Use vicious strikes to get your health back and to do more damage to players and towers.(if you gor more hp the demage will increase)
Reckless Swing one of the best skills vs tanks because this skill does true demage.
Ragnarok use it just when you are snared, exhausted... things that could stop you from killing the enemy.

Ghost and Exhaust.
Ghost, I think this combines with his ulti, if you use ghost and than your ulti you will be unstopable than you throw your axe to slow the target when you reach him you give him exhaust and you kill him.

You can use Olaf to solo but he is a good team player.
Use my firsts 3 important items and you can easly win a 3vs3 if the olaf is well managed.
And finaly think wisely before use the ulti.
Have fun playing and dont forget to vote.

5lyk