pick up undertow early for harassment and gank prevention, don't use to frequently because it will suck your mana.
VS (vicious strikes) helps keep you from having to back as much because of the lifesteal, and later on when you are stacked on health, VS does massive damage.
RS (reckless swings) is amazing for catching people unaware because it is true damage, meaning that is ignores negation.
Ragnorok is the binding point to this character. Wait until you are relatively low on health, pop ragnorok to become amazingly tough (as well as immune to disables, which helps with escapes) then VS for the dmg and massive life steal. Olaf's passive attack speed increase works very well with life steal.
Items: start by working for your berserkers greaves, movement speed is always top priority, health and damage are no good if you can't catch your prey.
then frozen mallet for the heals and the on hit slow effect.
after frozen mallet work for stark's fervor for attack speed and life steal.
Warmogs you could possibly work for before starks to give it time to build.
you are going to be tough to kill and have high survivability so sunfire capes help with HP and get in some good additional magic dmg
in the case of alot of casters, trade one sunfire for force of nature