Reckless Swing, is an intimidating skill early game. Anyone that can shield a part of it or heal back some of the health lost by activating a skill is a natural counter to Olaf. The champion should also match Olaf sustainability. The list is pretty long, but Rumble, Riven, Shen, Warwick and Jarvan are a few that come to mind. Just level the shield or heal first as a direct counter to Reckless Swing or until the lane phase ends.

Trololololol and an extra lol for the guys saying Fiora xD Vlad is perfect against him..

Fizz, His Q + E combo is great