Personally, I would have to say that, as far as I'm concerned, only what the Oracle says while in his trance should be taken as true prophecy. Information given outside the trance, though still possibly correct, is not officially part of the prophecy, and should not be treated as such. Therefore, the official prophecy, as I recall, was "Belkar will draw his last breath before the end of the year." or something like that. Any further information as to what this means was given outside of the trance, and therefore could be information he withheld from the official version, it could be conjecture, or it could even be outright lying. The Oracle has no real desire to be helpful, and for Roy, Durkon, and Belkar, he has reasons to want NOT to be helpful. Therefore, I would not count it as fact that everything he said out-of-trance will necessarily come to pass that way.

That being said, I do tend to lean rather more towards the side of "Belkar winding up on some other plane" rather than "Belkar comes back as an undead", but that's just because it seems like it'd be more in keeping with the style of the comic, but at the same time, it wouldn't surprise me if something else happened entirely.