I think the point, Mick, was how come the song is sung by England supporters. There are plenty of rugby songs available but why that one :-)
As jno says, the specific Chris Oti connection is disputed, but it seems pretty well documented that it started on the occasion of the 1988 England v Ireland match when England partly thanks to Oti's try-scoring exploits, scored an unexpectedly emphatic win, having themselves been trounced by Ireland the previous year