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.
the first four items are the core build. after those u can get whatever u want really, but i think youmus is very good for olaf because he will get an extremely high armor penetration. the blodthirster i think is good aswell because it gives him alot of extre damage and a ton of lifesteal when u pop his w spell. this build gives you very much durability and a very high attack damage.