Olaf is only good once you master him to a tee. If you mess up, you will call off and be useless. For me, building Pure life steal is the only thing that keeps me alive when I play Olaf ranked. If not, I die so much, and can't even farm without feeding.
Althogh I found that playing intelligently as Olaf counters Darius. Get as much life steal as possible, (Rav, thirster, Bork, youmuus core asap) poke, the q and e to win trades. Always use gurella warfare or you'll die.
If they would just buff the life steal on w, than tank Olaf would be a lottle more viable. I do terrible and die easy as tank olaf.