MTGNexus

A player with no other devotion to U in play and no cards in his library casts Thassa's Oracle. Would Jeskai Charm's first mode cast in response to Thassa's Oracle's EtB effect prevent the owner of Thassa's Oracle from winning when that ability resolves? It seems as if it should and apologies if it's as obvious as it seems and if it's been asked before.

It would indeed. While normally, removing Oracle doesn't do enough since Devotion of 0 is equal to the 0 cards in the library, you have now made their library have a single card in it. And 0 is no longer equal to or less than 1 so they will just look at Oracle and decide whether to put it on the bottom. Granted, this means they can still keep it in their library, draw it next turn, and have another shot at it, but it does allow for a bit more interaction to stop them.