The items listed above make up the core build and the optimal order of purchase, provided the game is fairly standard and you can rely on having the red buff for ganks/fights, thus postponing the need of Frozen Mallet.
A few examples of alternate routes:
If you're not facing any real CC or magic, get Berserkers greaves instead of Merc threads.
If the enemy CC and magic dmg is especially potent, rush Banshee's before Stark's fervor.
There are situations where Stark's fervor may even be less beneficial than other items, for example if you're the only player on your team benefiting from life steal and armor pen.
But regurlarly, Stark's fervor is an item that boosts every single stat Olaf needs and greatly empowers other AD carries on your team who should be coming hard into play at this stage of the game, thus making it a great item for the mid/lategame where teamfights are the in focus.
If you don't have a devoted tank on your team (hello solo queue) and the enemy team is AD-heavy, a randuins omen could be the item of choice after Frozen Mallet. If you for some reason can't have the red buff up for fights, get Frozen Mallet or at least Phage before Ghostblade etc and so on...
You get the idea. Always adapt. Open the Tab-screen and think.
Your job is to target the squishy damage dealing, Crowd-controlling son of a ***** at the back of their team in team fights. Pop Ghost + (R)agnarok and either A) Hunt down the bastard or B) Keep him away from the fight. Either alternative keeps that pesky Nuker (Anivia, Brand etc) from doing damage on your team and that usually gives your team a clear edge to win the teamfight.
Running a 0/21/9 build on Olaf is sure to raise an eyebrow or two. I get that.
When you think of Olaf you're thinking Juggernaut, right? The Unstoppable Force? The guy who charges through fire and then bites down on one player with Frozen Mallet, tearing him/her down with him to the grave.
Why shouldn't an anti-carry like Olaf take 21 points in Offense and make a fu**ing blast crater out of the enemy AP carry?
Well, here's why.
Like it or not, but Olaf's not quite Juggernaut enough to run through seven hells of nuke and still tear down an AP carry with crit proccs like Tryndamere, Gangplank or even champs like Caitlyn, Ashe or MF. He just doesn't work that way.
Building an infinity edge on Olaf is not something you get away with unless your team already is 15-3 in 20 minutes - and let's face it, you'd win those games building whatever you want. Those games do not count.
Olaf has the great benefit to ignore enemy CC and get up close and personal. But he will still die way too early unless he builds like a Bruiser.
The way Olaf's Passive, W and E work you want to have a deep health pool and a lot of damage mitigation, making you decepitvely strong even when nukes have brought you down below 40-50% HP. And you will often lose 50% of your HP in a split second at the start of teamfights running through the enemy team on your way to that AP carry at the back that is your job to take out. You need to survive that.
It is WHEN you've reached that AP carry (or whatever damage dealing champion at the back of the fight) that the fun begins. This is where the Terrier Mentality kicks in: Bite down on your designated target, spam axes and reckless swings while lifestealing your way back up.
When you're on 20-30% HP is when you're the strongest and those last 20% of HP should be as tough as possible to break all the way through. If you have enough max HP, armor and MR the increased attack speed from your passive combined with lifesteal will more often than you'd think keep you stubbornly afloat at 20% --> 30% --> 22% --> 27% --> 15% --> 25% --> 35% --> 55% --> 80% --> "Ace".
An Olaf who dies while running the Gauntlet or reaches his target with 5% is no good to anyone. The alternatives then become to A) either get a good flank, B) hope your tank soaks all damage or C) just fight other melee targets.
You need to be durable enough to ignore this.