Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
olaf has powerful abilities so to make him even stronger you should tank him. you should get health and then health regen
with olafs master abilities after grabbing some health you should go for some ap to up his abiliities even more.
finally we should get an attack damage and lifesteal maybe infinity edge and starks fevor or a bloodthirster or hextech gunblade.
attack with reckless swing when you can and then use vicious strikes and attack as much as you can finally slow him with undertow and grab the kill.