Thassa's Oracle represents the cleanest kill condition edh has ever had, and makes a variety of combos more slot efficient and more difficult to disrupt. Observe!
Thassa's Oracle is nuts. She represents a straight upgrade to Maniac, whose power was always somewhat checked by needing to finagle a draw effect. Oracle just immediately wins if you have an empty library, even if they remove her! And boy do we have some efficient ways to ditch our library!
Casting Thassa's Oracle and then responding to her trigger with Demonic Consultation, Tainted Pact, Hermit Druid, Divining Witch, and now introducing... Sacred Guide! (EDIT: and now also Mirror of Fate and Thought Lash! So many ways to get the deed done!)
Not only this, but these cards already comboed with the perfectly serviceable Laboratory Maniac... AND Jace. So now we have not only the most efficient combo package available, we're also among the most redundant. I'd say the main path to victory is trying to assemble this via Flash-Hulk, which conveniently grabs the whole package in a variety of ways.
The first combo you typically want to try and assemble is good ol' classic Flash-Hulk. Idk why they ever unbanned hulk, but whatever! In the past you had to do all sorts of bogus stuff like running 4-5 cards that did some stupid recursion combo or whatever... and we liked it! But now the kids are all:
Flash Hulk; Hulk gets Oracle, Sacred Guide, Lotus Cobra, Dryad Arbor, Blood Pet. Make white with Cobra off Arbor and sac blood pet to sac guide. This way you can't be stopped by spell hate.
And then here's some backup kills 4 ya:
Hulk into: Spellseeker, Thassa's Oracle, Blood Pet; find Demonic Consultation with Spellseeker then sac Boood Pet to Consult in response to the Oracle trigger. GG losers!
But wait, there's more!
If you have 1W available, a potentially EVEN SAFER line is: Hulk gets Sacred Guide, Oracle, Laboratory Maniac. Now you don't even lose to targeted draw! Also they can't counter guide! Great.
Here's some hulk piles if your opponent has Arcane Lab or one too many Thorn effects for ya:
Hulk into Laboratory Maniac, Hermit Druid/Divining Witch/Sacred Guide... options!; Flash on opponent's end step and you can untap and win with minimal response time.
Hermit Druid, Divining Witch, and Yisan all represent single-creature kills.
Hermit Druid can tap to ditch your library, then you can Dread Return Oracle. If that fails, you have Memory's Journey to set up 3 more attempts to win. Neat!
Divining Witch is slower, but on opponent's end step you Witch for Oracle, then on your turn cast Oracle and respond to the trigger with Witch FTW.
Thought Lash not only lets you lead the combo by resolving an exile effect, which doesn't exile until you choose; but with 98 card libraries and a lot of deck redundancy, this card can actually tank a lot of damage to play defense against things like other Tymnas or Edrics.
Mirror of Fate is an interesting addition; while it costs 5 mana, putting it at the tippy top of our curve; it offers a similar "resolve now and threaten to exile at will" while ALSO comboing with any other exile effect for a build your own doomsday. In fact, with Mirror of Fate and Memory's Journey, we can create a game state where we can threaten an increasing amount of kills per turn (but that's admitedly pretty niche; usually you can just use it to find something like Oracle//Jace//Memory's Journey and try to win twice, and then only get into that if you have to. Mirror of Fate also offers a sort of safety valve to recover cards exiled incidentally by Necropotence or opponent's effects or something, idk!
I guess I'll mention some typical Doomsday piles. Usually you can use Tymna to kick things off:
Cheapest:
(Needs UU)
Street Wraith > Gitaxian Probe > Thassa's Oracle > Jace > Memory's Journey
There's a bunch of variations; including ones that integrate Flash Hulk, but that's the main idea.
These combos are incredibly mana AND slot efficient, interlocking, and come out fast. Fastest is Flash. You can win turn zero if you get Gemstone Caverns and Spirit Guide and Flash and Hulk or Pact for Hulk or whatever. Good luck with that. Still, you can tutor into a turn 2 kill a surprising amount of the time. And all of this for the only filler cards being: Blood Pet, Memory's Journey, Thought Scour; and those are all "okay"; Pet can still accelerate, Scour can both help to draw topdeck tutored cards or mess up opponent's Vamps; Memory's Journey comically can stop opposing Oracles by putting 3 cards in their library! Thought Lash is actually an okay defensive card; people will be reluctant to attack knowing you'll prevent critical damage at will, and since the combo lines have so much redundancy, you can pretty much exile with impunity.