Maxing Q will give you more damage and sticking power during ganks, as stated below by another user.
Olaf is a champion who runs at someone with death being the only thing to stop him, which forces everyone to output their damage onto you or risk letting a carry die in a fight. To do this, you need to be very, very tank-y since your base stats are more than enough to kill a carry. If you want to watch someone play Olaf at a high level of play, TheOddOne plays him very consistently and he usually streams during the day.
My build for Olaf against a typical meta team (3 AD, 1 APC, support):
Cinderhulk, Righteous Glory, Ninja Tabi, Spirit Visage, Warmog's, Thornmail / Frozen Heart
Some situational items that I think you should consider are Randuin's Omen, Trinity Force, and Frozen Mallet.
Scaling Armor Yellows
Scaling MR Blues
AD Reds
AD Quints
21/9 masteries
The reason you don't take 9/21 masteries is because the only extremely useful mastery is Second Wind, and it's so far up that you have to take at least 17 in the tree. Your damage output is increased quite a fair bit by taking Spell and Blade Weaving, as well the capstone in offense. It is possible to go 13/17 for Spell / Blade Weaving and Second Wind, but it's untested how good this setup is.