Teemo is pretty good against any AD based melee fighter, but I can't say much for him, I don't play him often. Someone I do want to talk about is...
...Urgot, I know y'all seem to think he's all ADC all day, but his ability to poke, slow, shield, and ability to use Hyper-kinetic Swap even if Ragnarök is up, makes him pretty hurtful against Olaf's relatively short range kit and reliance on strong dueling rather than poking.
Cho'Gath is not a terrible choice either, he can keep Olaf at bay while he farms if Ragnarök isn't up. And can out sustain anyone in the game as long as he can last hit, which is to say, when ever he wants to. Once he starts chubbing up, he's very difficult to kill, and once he's at 6 stacks, his ability to burst down enemies while out sustaining them is extreme, especially if Olaf can't take down Cho while Ragnarök is up. The last note about him is... Zhonyas hourglass is viable on him, deal with that... :\
Jax is another good choice, one of the only champs who can go toe to toe with Olaf AND escape if needed. He also brings strong burst damage to the table, which is Olaf's biggest weakness.
Anyone who relies heavily on sustain and CC to duel as opposed to burst is pretty much screwed, however, as Olaf pretty much dominates that area right now. Fiora, Irelia, and Nasus, are some pretty bad note-worthy choices against him.