I have always thought IMHO that "folk music" is a music that is made by, felt by, carried on by, sung by: folks. Not some slick-haired Vegas crooner; not some manufactured "pop" teen icon; REAL people who are the same in performance as they are at home. Oasis qualifies; Anyone with the chutzpah to call George Harrison an old fart and risk the wrath of 100 million middle-aged housewives is definitely down to earth enough for me! (Not that I dislike 'ol Harrison; I LOVE his playing and writing) I believe that the "folk" in the person determines the "folk" of the music.
Peace on you,