Olaf is by no means broken. Many champs may seem op if played in right hands or not looked at through the right eyes. As a D3 Olaf main Olaf has many strengths and weaknesses. He has strong early game but trades that for a weak late game as he falls off pretty hard compared to most champs in his similar role ( jax, Trundle, Irelia) He provides a unique roll as he is good against comps who are immobile or have lots of cc. But the thing is he is not a super tank and can be blown up when focused by good coordinated players. Olaf's passive and life steal are only good for laning phase outside of it you hardly abuse the attack speed you get from being low as no one is going to stand and slug it out with you, same with his life steal its good for lane but terrible for team-fights.
Olaf is a champ that is decently balanced around strengths and weaknesses, he loses out in some areas and gains in others. Op to me means they excel in all aspects such as graves was in past being tanky and doing lots of damage throughout the game. It also depends on the champ you are playing dont expect to walk up as an adc by yourself and expect to live.