I'm not sure how this works. I mean, it does look like fate is fixed (no preventing the Oracle's prophecies from becoming true) and it doesn't look like the Oracle can see things that won't actually happen, i.e. he can't foresee Xykon killing him in a way that a Resurrect spell can undo and thus decide to not be there, because in that case the temporal line wouldn't feature Xykon killing him and he wouldn't have been able to foresee it. How much agency he has is an insoluble metaphysical problem that has troubled philosophers and theologians for millennia, so the message board of a stick figure comic is unlikely to provide a satisfactory answer.

So let's just say that it be like that sometimes.