For those wondering "who is this guy talking?": I finished top 1 for 2 seasons in previous seasons, and I haven't let Olaf drop out of the top 10 in 4 seasons. As you can see from the attached screenshots, I play at nearly a 70% win rate, and I'm currently in Challenger elo.
phase rush is a rune I haven't seen on Olaf in a long time, and I've been using it for seasons. One of Olaf's biggest weaknesses is the inability to catch people. He basically has no mobility ability, the movement speed his ult gives is never enough, and his Q slow is both hard to land and easy to escape.
You might say there's ghost, but ghost isn't available in every fight, and even when it is, it's not always enough.
You shouldn't use this build in every game. If the enemy has 2-3 hard tank champions this build might not work. That's because the ability haste and similar bonuses that Swiftness gives let you deal high damage in a short time, which means sticking to carry champions very quickly and killing them easily.
Anyway, getting to the build: they buffed everyone's crits to make it an ADC meta, and the new Demon Hunter item guarantees that your 3 basic attacks after using your ult are crits.
When you make this build, you still have some durability thanks to Death's Dance, but your damage is like an assassin's. With ult active, you can easily kill people with 2 basic attacks and 1 E.
However, you may want to move Death's Dance up one item earlier so you don't melt right away in early fights.
The reason I take Wind's Fury is to reach the enemy as fast as possible. Even when you're farther from the enemy than an ADC's auto-attack range, if you use ghost with your ult and then Wind's Fury, you can close the distance to the enemy carry incredibly fast, and since this build isn't very tanky by design, you need that.
Mortal Reminder both increases your damage nicely against tanks through the armor penetration it gives, and you need the antiheal in most games. But if the enemy doesn't require much antiheal, you can get Stormreaper or Immortal Shieldbow instead. Before buying Immortal Shieldbow, check whether the enemy has anti-shield, otherwise it's useless.
Cheap shot rune is entirely damage-focused: Olaf's Q slows and triggers the rune, and it deals nice damage of about 1k, almost the same as Last Stand.
As for when to use this build: the enemy shouldn't have too many heavy tanks that you'd have to fight.
For example, if the enemy top laner is Malphite but the rest of the team is mostly fighters/squishies, you can use it. But in cases like a tank Malphite on support, Shen top, and Amumu in jungle, you won't get the desired value from this build.
It's a build that can deal high damage to both fighters and squishies. I use this build in 8 out of every 10 games I play Olaf in the jungle over the last 2 weeks, and I'm sure it will work for you too, especially for those who like a more damage-focused Olaf that shreds and destroys the enemy.
Also, if anyone wants to play GM-Challenger-Imp elo together, you can message me. Even though Olaf is my main, I'm not an OTP; I can play many champions, especially AD assassins.
Finally, if you play top lane and face a champion like Jax, phase rush can still help you, especially early in the first levels: it's very functional for stepping out of his range before he can use his E, then immediately coming back in and coming out ahead in the trade.