olaf is a tank champion who have huge scary life steal and he can be more dangerous when he is low hp because of his passive, his passive can give him attack speed 1% for increased 1% missing health.

this berserk is one of the best in 1 vs 1 who can win most of the champs early or late in jungle.

most of champ can be dangerous for olaf are xin zhao, j4, trundle, jax, matser yi and yorick
in early game with out ulti its hard to can fight with xin zhao or j4 or trundle but still have chance to win but others champs it depends on your play to win against them.
champions like yorick and j4 can hold olaf and olaf needs to move to take his axes its betetr to dont fight with them without flash spell.

there is item can counter olaf like ninja tabi randuin's omen, dead mans'plate, executioner's calling and thornmail.
but most of these are not helpful in late game if enemy dont take bouns health because of his true damage on E.

olaf q: when you are ganking a lane most important thing is to dont miss your first q if hit q to the enemy you can take your q again and most of time enemy cant run from it even with flash
olad W: try to use W in heavy trades, not quick ones. if you aren't getting most of the duration off in autos, probably isn't worth spending mana. you can use it to sustian too.
one of the most things is most of players use ragnarok when they are on low hp its true but its not worth risk on your hp and when you are trying to gank before getting cc like syndra E to push you back use it