in the last few tournements, olaf was banned/picked in every game, he was quite op in the right hands.

Well that's like the top 1% of players lol. I guess Olaf would be quite scary in that range of skill but so are alot of champions in the hands of a professional player. I'm not sure how applicable this is to the whole of League of Legends. I mean in tournaments they don't just ban champions because they are OP, they ban them because they mess with whatever plan they have in mind / to gain an advantage over the other team.

A champion can be OP and have a low win rate, because people don't know how to play him.

Its called the bandwagon (Though other factors do contribute)
Olaf doesn't get BC according to most, but it still does affect him.

He was already capable of this since season 2 where he remained untouched for so long.
His jungling was ok, his top lane was decent, but people didn't see why he should be picked over other tops.

He was built CDR tank but no one really paid attention. They were too busy complaining about Rengar and Diana.

Now that a pro player builds CDR tank on Olaf
SUDDENLY he is op and needs nerfs.
It is because of this hp stacking meta where Olaf shines and doesn't get held back by it.
His W does more and his Passive kicks in more effectively.
In addition everyone is so scared of his true damage they forgot that the rest of Olaf exists.
Thats why they nerfed the cd and Q.
Personally I think it is dumb (and no I don't play Olaf)
He is manageable so long as you aren't dumb and feed him horribly (which isn't hard if you can dodge skillshots). and you can out harass him pretty easily despite the true damage (I've done it before with Renekton and Darius... ok ignore the last one)

Despite that people keep thinking he is op because they run into him range and get minidunked and get all scared.

Trust me bandwagoning is inevitable.
One day Heimerdinger will be considered OP when some pro does good with him.

As for the HP stacking meta going on right now
INB4 Sion is considered OP

He is one of the only bruiser champs who didn't get hurt by the season 3 changes and so he is very strong right now is all. Not OP, just strong.

He is strong and maybe op. He does incredible damage without damage item. Its funny though Xin Zhao is far stronger ATM with way better ganks. Also he destroys Darius, which someone up at riot loves.

But the nerfs really dont matter, if you hit max CDR you will still do insane damage with 260 DMG Q's for nearly 900 DPS @ .3 second cd with no bonus damage. That's what's making him too strong. The armor pen change will make armor slightly more effective but not really lol. He is a super fun champ to play.

He still lacks any cc and can't peel, but he can initiate like a boss. Singed with liandrys and rylais late ruins his day.

he was like 100% pick/ban in a recent event and he did really well. he does everything, and he does it well.

reckless swing is true damage and has an extremely high base damage making it perfect for lane.
undertow is a perma-slow
he even gets +%dps from his passive and ult, in addition to defense

warmogs change probably didnt help things.

He gets more pro play than almost any other champ. Though i think health needs a nerf not olaf.