Olaf can combine Berserker Rage, Vicious Strikes, and Ragnarok at low life to become deceptively strong.
The bonus healing granted by Vicious Strikes amplifies your Life Steal from all sources as well as heals from allies.
Olaf becomes more dangerous the lower life he is. Save your disables to finish him off.
Preventing Olaf from reaching his axe will minimize the amount of harassment he can cause in the laning phase.
Olaf has reduced defenses from damage during Ragnarok, despite being immune to disables. If you can't escape from Olaf during Ragnarok, try focusing your damage on him with your teammates.