Solution
Here comes the solution to this year's Christmas puzzle:
First, is the story in the museum true or false? The crucial question is whether the last sentence in it is true. It goes:
*) If the story is true, the oracle finds out that it is.
Under what conditions is (*) false? It is false iff i) the story in the museum is true, but ii) the oracle doesn't find out that it is. On the other hand, since (*) is part of that very story, if (*) is false, the story is also false. So if (*) is false, the story is both true and false. So (*) can't be false.
Now all the sentences of the story except (*) are repeated in the outer story (the puzzle fiction), so all the other sentences are true as well (in the fiction). So the entire story is true.
Finally, from the truth of (*) together with the fact that the story is true it follows by modus ponens that the oracle finds out that the story is true. Which answers the question.
I now realize that one might question whether for (*) to be false, the story must be true. Maybe I sould have replaced it by "either the story is false or the oracle finds out that it is true."