Well, the obvious answer is that GG Oath doesn't win the game the turn you activate Oath, unless you're lucky enough to have a Time Walk in hand. So by being able to win without passing the turn, either by storm (like this deck and Tyrant Oath) or by chaining Time Walks and/or assembling Vault-Key (like RSD Oath) you speed up the clock by a whole turn, which is going to have a pretty big impact on how your matches play out. I mean, thanks to Phyrexian Metamorph, even the Workshop decks that would have been cold to BSC or Emrakul a few months ago now have a very good answer to both of them if you give them the time to play it, and there are plenty of other decks that can convert that one extra turn to a win by comboing out. Also, Jace is still incredibly popular and acts as a very good answer to anything GG Oath can pull out of the bag.

Also, in the case of this deck and the RSD Oath deck that Mark Hornung wrote about recently, the extra combo means that you don't have to go through Oath to win, so you can win a lot of games where Oath itself would traditionally be a very poor strategy. In this deck, certainly, Oath is just the most powerful of a couple of different engines that can all lead to storm kills, whereas in Mark's deck Oath is a powerful engine for setting up Vault-Key (and Demon-beats) but he also has JTMS, Enlightened Tutor and the capability to just draw into them naturally.

As someone who loves both storm decks and decks that abuse the graveyard, I immediately started brainstorming ways to abuse Past in Flames when it was first spoiled. A lot of people were talking about finding a shell that could use it as extra virtual copies of Yawgmoth's Will, but I wanted to find a deck where it could actually be - at least some of the time - better than Will. Eventually, I remembered a deck concept that would really benefit from this card, a deck written about by Aaron Forsythe here: http://www.wizards.com/magic/magazine/Article.aspx?x=mtgcom/daily/af39

The basic idea of the deck was that it was a hybrid Oath/Storm deck that used Oath as its primary 'draw' engine - you Oath for Eternal Witness, filling your graveyard with some goodies in the process, regrowth whatever the best card in there is and hopefully win from there. If that card happened to be a Yawgmoth's Will you could probably win without passing the turn. Anything else and you might be able to depending on what was in your hand. It was a really cool idea but it seems like it just wasn't consistent enough relative to other storm engines.

With Past in Flames, however, you can take that strategy and make it much, much more consistent. You probably don't want 4, but if you have 2 or 3 then your chances of getting either PiF or Will into your graveyard when you Oath go up drastically. In fact, Past in Flames is often going to be a BETTER play than Will because if you have one in your graveyard, you can Witness for Black Lotus or Lion's Eye Diamond, if you have one, and then cast it for its flashback cost ( ), which is a very similar play (if you've built your deck with Past in Flames in mind) to casting a Yawgmoth's Will that only costs 2 mana, leaving you more mana to cast counterspells or rituals than you would have if you had pulled an actual Yawgmoth's Will back to your hand. Of course, having the Past in Flames in your deck, alongside the Yawgmoth's Will, means that you can also assemble storm kills without going through Oath first without too much difficulty.

So, I started brainstorming and goldfishing some games and came up with quite a few ideas, which ultimately resulted in the deck below. One important note before addressing the auxiliary card choices: although I started with Eternal Witness as the Oath target, I soon realised that Rune-Scarred Demon was going to be just better in this deck, which also led to the name, DemonFire. Witness looks like it might have some fringe advantages in that you're more likely to be able to hardcast it, but realistically I think there are very few situations where you'd both want and be able to do this. Demon's advantages, on the other hand, are much more significant - if you get really unlucky and hit Demon too early, meaning there aren't enough cards in your graveyard for a good Past in Flames or Yawgmoth's Will turn, or too late and you can't search out any good targets, then you can sometimes shift your gameplan to counter their key spells while attacking with Demon for the win. (Note that if you really need more cards in your graveyard you can always wait a turn and let Oath put all the other cards in your deck there - also, the analysis earlier about Past in Flames increasing consistency still holds true because after you Oath you should have a similar number of cards in your graveyard and your library, so if you only have Yawgmoth's Will there's a chance you won't be able to search for it because it got put in your graveyard when you wanted it in your library).

Anyway, here's the first (well, 1.1th) draft of the deck:

DemonFire

Non-ritual Mana (22)
4 Forbidden Orchard
4 City of Brass
1 Tolarian Academy
3 Gemstone Mine
1 Black Lotus
1 Sol Ring
1 Lotus Petal
1 Lion's Eye Diamond
1 Mox Pearl
1 Mox Ruby
1 Mox Emerald
1 Mox Jet
1 Mox Sapphire
1 Mana Crypt

Rituals (5)
4 Dark Ritual
1 Cabal Ritual

Oath Package (5)
4 Oath of Druids
1 Rune-Scarred Demon

Card Draw/Selection/Tutors (14)
1 Vampiric Tutor
1 Imperial Seal
1 Demonic Tutor
1 Noxious Revival
2 Gitaxian Probe
1 Time Walk
1 Ancestral Recall
1 Brainstorm
1 Ponder
1 Sensei's Divining Top
2 Preordain
1 Necropotence

Protection/Disruption (depending on matchup) (10)
4 Force of Will
1 Hurkyl's Recall
2 Mental Misstep
2 Flusterstorm
1 Cabal Therapy

Endgame Cards (4)
1 Yawgmoth's Will
2 Past in Flames
1 Burning Wish

Sideboard (VERY rough)
1 Blightsteel Colossus
1 Terastodon
1 Tinker
2 Duress
1 Tendrils of Agony
1 Empty the Warrens
2 Ancient Grudge
2 Tormod's Crypt
3 Pithing Needle
1 Nihil Spellbomb

So that's the deck and a basic explanation of the strategy, now a brief overview of my thoughts behind the choice of cards.

Firstly the mana - the deck's manabase is not great, I know. Absolutely no resilience to Wasteland apart from artifact mana isn't a good place to be, particularly when your land count is as low as this deck's is. but at least the deck makes up for it somewhat by not needing that much operating mana due to the cheapness of its primary engine and the number of free spells it packs in. Given that you're definitely packing 4 Forbidden Orchards and probably a Tolarian Academy, and you need green for Oath, red for PiF, black for rituals and necro and blue for counterspells and library manipulation, I don't really think that a traditional fetcland/dual-land configuration is viable. As for the artifact mana itself, the basic idea is to make it as easy as possible to win from a big Yawgmoth's Will. Mana Vault may have a place but it doesn't seem necessary, especially when you consider that at the moment the primary mana bottleneck usually comes early in the game rather than later on. I do feel the deck's a bit mana light but I'm not entirely sure what to cut. Also, I know that Past in Flames doesn't actually let you cast artifact mana from the yard but I've found that one of the most common plays is to cast a Past in Flames for the mana and the manipulation to be able to find and cast a Yawgmoth's Will so you can build your storm up enough to kill them. (BTW, if you're looking for other directions to take this deck, what about Crop Rotation and Bazaar of Baghdad as a slow draw engine since you're basically going to be living out of your graveyard most of the time and Crop Rotation would also let you find Forbidden Orchard more consistenly)

The rituals are good both before and after a PiF or a Will, but they're certainly more necessary here than they are other decks that use Yawgmoth's Will since they'll be your primary source of mana after you cast Past in Flames. I did consider the red rituals, given the fact that Past in Flames itself needs red mana, but they don't do nearly as much before you cast Past in Flames as the black ones do, and they don't really let you build a deck that can win like a regular storm deck. Also, having Dark Rituals lets you run Necropotence, which gives you the potential for some ridiculous 'I win' hands and acts as an incredibly powerful back-up engine.

The topdeck tutors (Imperial and Vampiric) coupled with the Gitaxian Probes are very powerful here, and will often be your way of finding the kill once you have plenty of mana in your graveyard (along the lines of flashback some rituals, flashback Vampiric Tutor for Burning Wish, flashback Probe for 2 life to put it in your hand). Ideally I'd play more Gitaxian Probes (partly because I just love that card) but in testing the deck was proving inconsistent at setting up Oath and Forbidden Orchard so I cut 2 for Preordains. Also, the Top was added because it's generally a good card in Oath decks and because I can imagine situations where you'll want to set up a long game involving tutoring for it, but I could certainly see cutting it for a 3rd Preordain. Brainstorm loses a lot of its value without fetches but it's still very good in Vintage, even without them: however, you need to be careful. It's often worth conserving it if you have other plays so you can combine it with a tutor for maximum value. The Noxious Revival is there to help make up for the inability of PiF to work on artifact mana, since you can cast it to put Lotus or LED on top of your library and then draw it with a probe to be 3 mana up or another cantrip to be 2 mana up. It also increases your resilience to counterspells, since if you're planning on flashing back a Yawgmoth's Will or a Burning Wish you can put it on the top of your library first so you get two chances at casting it. It also just has a whole load of utility should you draw it early in the game, from blanking their draw-step to being a virtual counterspell if you have Top out. Also there are plenty of times where you just Revive a cantrip so you can dig some more.

The disruption is very rough at the moment, and the blue count (17) could definitely be higher to support Force better, but as the premier counterspell in the format, it feels like it should be in here. Increasing the blue count should definitely be looked at (an extra Preordain or two maybe). Flusterstorm is good here because it's a maindeck answer to fast combo decks, which seem like they could be a major vulnerability for this deck, that also has applications in other matchups. Mental Misstep is generally pretty good in this meta, and in this deck it can also generate a bit of extra storm if you have them lying around in your graveyard. The singleton Cabal Therapy is the best way of adding protection to the Oath 'kill' since you can sac the demon to it if it goes into your graveyard, and it also helps in that there are definitely situations in which you want to get rid of your demon. The Hurkyl's is, as in other storm decks, both a powerful defensive tool and a potential storm engine.

The actual kill most of the time will be Yawgmoth's Will and/or Past in Flames building to a lethal Tendrils via Burning Wish. Past in Flames is good because, as mentioned earlier, if it's in your graveyard you can tutor for Lotus or LED and save mana, and it will often give you the tools you need to find and play a Yawgmoth's Will. Will is good because, well, it's Will. Of particular note is the fact that it enables kills that don't involve Oath, particular when it feels like working with Ancestral and Necro. The maindeck kill was originally Tendrils of Agony but I found that Tendrils was just a dead card right up until I wanted to use my graveyard to kill them, at which point I had more than enough mana. I changed it to Burning Wish, which, at that point in the game, is just a more expensive Tendrils (which is fine, because you always have more than enough mana) but is MUCH more useful in the early game, since Empty the Warrens for 4 or 5 is a perfectly valid strategy against a lot of decks. If you can, you'd rather kill with Tendrils, obviously, but Burning Wish is useful in many more situations than Tendrils is. Having said that, the addition of this avenue of attack may not be worth the cost of 2 sideboard slots, but I think it's definitely worth trying. Also, without Burning Wish, Lion's Eye Diamond loses a lot of its early-game utility, but you can't cut it because you need it as a backup Lotus for casting stuff from your graveyard.

As for the sideboard, it's definitely one of the best things about playing this sort of Oath deck - in matchups where you just want to be a traditional Oath-fatties deck, you can turn into that fairly easily by bringing in Blightsteel, Tinker and Terastodon (although the Terastodon may well be overkill). This also lets you play mind-games with the opponent, much like Minus 6 does, where they become uncertain of whether to bring in graveyard hate or not. The rest of the sideboard serves some fairly obvious purposes - I really wanted to fit some Duresses in the maindeck but I just couldn't, so I put them in the sideboard where they can come in against the most disruption-heavy decks and the really fast combo decks. The Pithing Needles are mainly for Dredge but also help address the fundamental weakness to Wasteland.

Anyway, that's the deck. As you can see, it's still very rough at the moment and feedback would really be appreciated. Could the mana be better? Should I push it towards being more controllish or more consistently fast? Is Rune-Scarred Demon definitely better than Eternal Witness? Can the blue count be increased? What's the right number of Past in Flames? Is DemonFire actually a better name than The Perfect Oath? Does the deck actually have potential or is just inferior to other Oath strategies?

Thanks for reading!

UPDATE: In testing, it looks like 2 Past in Flames might be overkill. Having 1 is really good, and definitely makes the deck MUCH faster and more consistent than it would otherwise be, but the 2nd one probably doesn't help that much and would probably be better as a 3rd Preordain. Also, I've been considering cutting the Forces, since the blue count is fairly low and you don't have a lot of card draw, so the card disadvantage hurts quite a bit. Maybe replace them with some mix of Duresses and Beast Withins (good for dealing with  troublesome permanents while also turning on Oath of Druids)? Thoughts?
Also, I'm now virtually 100% sure that the SDT is not pulling its weight without fetchlands in the deck. As such, I need to either replace it with a Preordain or cut the Citys and Mines for 4 Misty Rainforest, 1 Sea, 1 Volcanic and 1 Tropical, which could lead to some very awkward hands that would otherwise have been keepable. The former option makes Force better but the latter adds utility to Brainstorm as well. Of course, I could do both as well. Having goldfished a few games, the strain on the mana isn't that bad - however, what is bad is that it makes it a LOT easier for Wasteland to wreck you, since before they couldn't cut you off an entire colour that easily.

With that in mind, this is the list I'm currently at:

Mana (27)
4 Forbidden Orchard
4 City of Brass
1 Tolarian Academy
3 Gemstone Mine
1 Black Lotus
1 Sol Ring
1 Lotus Petal
1 Lion's Eye Diamond
1 Mox Pearl
1 Mox Ruby
1 Mox Emerald
1 Mox Jet
1 Mox Sapphire
1 Mana Crypt
4 Dark Ritual
1 Cabal Ritual

Game Winners (8)
4 Oath of Druids
1 Rune-Scarred Demon
1 Past in Flames
1 Yawgmoth's Will
1 Burning Wish

Card Draw/Selection/Tutors (15)
1 Vampiric Tutor
1 Imperial Seal
1 Demonic Tutor
1 Noxious Revival
2 Gitaxian Probe
1 Time Walk
1 Ancestral Recall
1 Brainstorm
1 Ponder
4 Preordain
1 Necropotence

Protection/Disruption (10)
4 Force of Will
1 Hurkyl's Recall
2 Mental Misstep
2 Flusterstorm
1 Cabal Therapy

Sideboard
1 Blightsteel Colossus
1 Terastodon
1 Tinker
2 Duress
1 Tendrils of Agony
1 Empty the Warrens
2 Ancient Grudge
2 Tormod's Crypt
3 Pithing Needle
1 Nihil Spellbomb