I'd like to know which one Olaf mains prefer, or, if both are equal, in what situations would one be better than the other?
Also, random question, what "subclass" do you think he fits under? Dive-Fighter? Juggernaut? A bit of both?
If you're playing a melee top, grasp of the undying is the way to go.
i personally like Grasp or Warlords, I'll either go 12-0-18 or 18-0-12. I like the extra healing when i can auto attack, some fights you dont get to auto much at all if you are chasing with Q's you tend to throw in E's when you can and then you have to wind up another Q.
Subclass? Probably Juggernaut.
Grasp of the Undying. Sometimes, I feel like 
is only viable as a full tank.
Warlords x 100. Even Riot has told you to take this on him:
Greetings, Summoners
Welcome to Patch 6.4, the one where Warlord's Bloodlust is useful.
“But I love healing in combat!” you might say.
To which we'd respond, “Just wait til' you get a load of it on Olaf!” We'd then ask why you’re talking to your computer (or mobile phone), but there’d be no response because we’re all just figments of each each other’s imagination.
And that's it for us! Check out all of the patch note goodness below, and we'll see you on the Rift, taking Warlord's Bloodlust on every champion and living like a true viking.
As is, Warlord's Bloodlust has two main problems. For starters, it's incredibly narrow - while certain Keystones might restrict you to attacking so often or doing certain thresholds of damage, Warlord's specifically requires you to crit. While being narrow isn't always bad (Windspeaker's Blessing is fine being taken by supports only), the heavy skew towards marksmen when Fervor and Thunderlord's are also great options make Warlord's superfluous (particularly when other classes could benefit from the space).
Aside from its intended audience, Warlord's has always captured a lategame fantasy while giving you nothing immediately - a tradeoff that people just aren't excited to make. These changes take a crack at killing two birds with one key-stone. Removing crit broadens its application significantly, making Warlord's especially synergistic with 'all-in' fighters like Aatrox and Olaf that capture the spirit of the Keystone's concept and giving it more reasonable scaling over the course of the game. If you like to fight to your last breath (and then some), look no further. NEW: FOR THOSE WHO FIGHT FURTHER Attacks on champions gain increasingly more Lifesteal based on your missing health (up to 20%). Half effect vs minions.
fervor, nobody good goes Warlord on olaf
http://champion.gg/champion/Olaf
Looks like, of the two choices you gave, Fervor is the superior one.
Grasp or Warlord's, and he is a diver.
Grasp if going X-X-18, or Fervor if 18-x-x, but I'm not too experienced with Olaf, so I wouldn't know. I'd assume he has enough natural sustain to not need Warlord's sustain, so.
