What happens if a player runs out of cards in their library but has a Thassa's Oracle trigger on the stack?

When Thassa's Oracle's triggered ability resolves, it checks two things: first, you look at the top X cards of your library where X equals your devotion to blue, then you win the game if your library has fewer cards in it than your devotion to blue. However, the win condition actually reads: 'If your library has no cards in it, you win the game.' This is the critical clause — an empty library alone triggers the win condition on resolution.

Under CR 704.5b, a player who attempts to draw from an empty library loses the game as a state-based action. However, state-based actions are only checked at specific times, not while a triggered ability is on the stack waiting to resolve. If the library becomes empty before the Oracle trigger resolves (e.g., due to mill or self-mill), the state-based action loss is checked, but a player can respond by allowing the trigger to resolve first if it was already on the stack.

The key interaction: if Thassa's Oracle's ETB trigger is already on the stack when the library empties (or was empty when it triggered), the trigger will resolve and check the library size at resolution time (CR 608.2b). At that point, if the library is empty, the win condition is met regardless of devotion count, because the Oracle's text says 'if your library has no cards in it' as a separate clause from the devotion check.

Concrete example: You cast Thassa's Oracle with 0 devotion to blue and an empty library. The trigger goes on the stack. Your opponent attempts to make you draw a card — but before that resolves, the Oracle trigger resolves. On resolution, your devotion is 0 so you look at 0 cards, then the game checks: does your library have fewer cards than your devotion? No — but it checks if your library has no cards at all. It does, so you win the game immediately.

Important caveat: if a state-based action loss (CR 704.5b) is processed before the trigger resolves — for instance, if priority passes and SBAs are checked while your library is empty and you have no Oracle trigger on the stack yet — you would lose before winning. Timing of when the library empties relative to the trigger being placed on the stack is crucial (CR 117.5, CR 704.3).