There are more than one "correct" set of chords for almost any song. Some songs can be accompanied by as few as one (or no) chords.
One of the first songs I learned to accompany was Burl Ives' version of a song I called Jock Of Diamonds. I used two chords for this song. Soon after I heard Jackie Washington play the same song and he used two chords per measure.
Most people would use 2 or 3 chords to accompany Oh Susannah, but James Taylor's version uses about two changes per measure and many more than two or three chords. GUEST,number6's mention of James Taylor reminded me of this.