How to counter Olaf
Let us take a look on how to beat Olaf. We will look at champions who are strong against Olaf and who are weak against Olaf. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Top champion who counters Olaf. You can already win in champion select with the right counters for Olaf. If you want to know how to counter Olaf then you need to take into account the best picks vs Olaf and the worst picks vs Olaf. This Olaf counter data is based on matches played by Pros on patch 15.12 and previous ones. It is important to see whether Pros counter picked Olaf or whether they played as Olaf.
Champions used by Pros to counter pick Olaf
Looking at the matches from Pros (see table Pros counter picked Olaf) we can see that they picked 
Gnar most often to counter pick Olaf. The matchup Gnar vs Olaf occured 13 times. Olaf has 9 wins and 4 losses in this matchup. Learn how to counter pick Gnar to get another perspective on the data.
Lane outcome for Olaf
When looking at Olaf counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Olaf. The lane outcome of Olaf consists of three outcomes: Win, Loss or Draw. Let's take a look of Olaf lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Olaf.
Conclusion
Analyze the available data and learn how to beat Olaf. To master Olaf take also a look on Olaf builds and runes for Olaf. Good luck, Summoner!