A 2-mana blue Merfolk that wins the game outright if your library is empty when it enters — which is exactly what Demonic Consultation and Tainted Pact are for.
The Thoracle combo. Demonic Consultation or Tainted Pact to empty the library, then Thassa's Oracle to win off the empty library — one of the cheapest, fastest kills in Commander.
Mono-blue or Simic devotion. Its scry ability scales with blue devotion, giving it a real job in a deck that isn't going for the combo at all.
Backup wincon in control shells. A control deck that's already drawing and tutoring heavily can find itself with an empty library by accident — the Oracle turns that from a loss condition into a win condition.
How it does in Commander
As the commander
It's not commander material — a 1/3 for two mana with a conditional win trigger doesn't want to be the one card you always have access to, since drawing it early with no way to empty your library does nothing.
In the 99
This is the card's entire job: a two-mana insurance policy and finisher in any blue deck running Demonic Consultation or Tainted Pact, and a real threat that forces the table to hold up interaction the moment both pieces are on the battlefield or in hand together.
Formats it's strong in
cEDH and other fast, high-power pods. A same-turn two-card kill for three or four mana is exactly the speed those tables are built to answer or race.
Any blue deck already running Demonic Consultation. The combo piece is often already in the deck for its tutoring effect alone, so the Oracle is a low-cost addition.
Formats it's weak in
Casual or precon-level pods. A hard combo kill that can come down before turn three is a rough surprise at a table that hasn't discussed power level — worth a Rule 0 conversation first.
Decks with no library-emptying effect. Without a partner, it's a below-rate creature whose upside never comes online.
Synergistic cards
Demonic Consultation — The classic partner — one black mana to exile your library, naming a card you're not running.
Tainted Pact — The blue version of the same effect, exiling instead of putting into the graveyard, for decks that want to stay off-color of black.
Jace, Wielder of Mysteries — A second, redundant "empty library wins" body that isn't a creature and dodges creature removal.
Laboratory Maniac — The original version of the effect, for decks running it as a fourth or fifth copy of the plan.
Is it worth the price?
Thassa's Oracle stays cheap on its own; Demonic Consultation carries most of the combo's price. As a two-card kill that fits almost any blue deck, it's worth owning even before deciding how many redundant copies of the plan to run.