Not Updated For Current Season
This is the build that I use personally for Olaf and I feel that it is an excellent build. Firstly it focuses on attack speed and armor penetration, along with armor and health to help survivability at lower levels. Then life-steal and attack damage is focused on as it is severely important along with unique auras to decrease armor. Finally, Olaf needs to slow enemies to bring his effective ganking abilities to the full and help his survivability so I chose the Frost Hammer.
Olaf's runes are a mixture, mainly focusing on attack speed but also on max health and cooldown, all very useful attributes when playing as this character.
Olaf focuses on Undertow and Vicious Strikes throughout the lower levels, with the Ultimate being learnt as soon as it could possibly be learnt. Reckless swing isn't as important at the lower stages as undertow can do effective damage but towards the end of the match you need to knock off huge chunks of enemies' health and reckless swing is the best for that job.
Revive is vital for Olaf especially with the revive mastery simply because if you die you can revive and take the enemy out quickly before they can turn the tide too much. With his increased HP, it will be doubly hard to take him down.
Exhaust is useful when taking out enemies with more DPS like Master Yi as it disable's their main damage long enough for you to escape or destroy them.
As I have said, I use this specific build when playing as Olaf and I normally am a major contributor in a match, if not the man-of-the-match. Olaf is known for his strength at higher levels so make sure you don't get too cocky and take several champions until you reach those levels (especially without your ultimate at least a few items). Enemy champions will try and stop you from leveling too much as Olaf is one of those champions that are superior if they are higher levels than you.
I hope this build and my advise helps, and have a nice day :)