Riven vs Olaf was always a skill-lane more than a true counter, if you dodged his axes and used your 3rd application of Q together with stun just as Olaf used W (his lifesteal-steroid)you would come out ahead in almost any trade, shielding E (true dmg nuke) was style-points!
OnT: If you are a strong Riven player you can win against pretty much anything.
My only problem with Zac top is that you are actually very weak until you have backed atleast once, against a bruiser reigning between levels 1-5 (Riven, Jax, Lee etc) you would be at a instant disadvantage. However, Zac brings alot of cc to teamfights and if you play your early-game accordingly it shouldnt be too bad afterall.
Fast edit: I don´t want to sound mean but isnt infact "overtuned" just another way of saying "OP" ?