I don't think "Best Original Song" negates the heuristic. Generally when someone talks about "Oscar Worthy" they are generally considering the original categories or even just best picture, director, actress, actor, etc. Maybe best original screen play added in 1940. Not to diminish the performers or writers of the Best Original Song, it's still an awesome achievement. Just more personal than something like Best Picture.
>>davegu+(OP)
Yes, I agree. Put another way: It's hard for a movie with the best director or best screenplay to be a bad movie, but it's very easy for a movie with a great original song to otherwise be terrible.