This is how i like to build Olaf, as a off tank high DPS life stealing son of a b*tch.
Pop that ult and slow them with your mallet = Secured kill.
I pick ghost over flash because of the fact that u have your ulti to help you escape if stunned or slowed. and ghost can bring u to a greater distance depending on the direction your running.
I start out with vamp scept because it gives me an easier time on my lane.
In 5v5 solo top i max W since its added life steal to your 12% giving you a stronger lane chance and 1v1 battle can change quick because of your passive.
Late game you are nearly invincible 1v1. You cant get stunned since if you use your ulti correct, and they cant escape since you have ur frozen mallets passive. And if the flash to a location that you have vision over. you pop ghost and run in range for either a basic hit or a Q.
You serve as a great pusher as well, with good escape chances.
3v3 is basically the same. Though u are weak against some ranged harrasers like Nunu or Teemo.