What's the difference between ο φίλαθλος and ο οπαδός? Maybe the first is more specific for sports and the second is more general?
True, "φίλαθλος" can only refer to sports, whereas "οπαδός" could refer to other stuff as well. But you could say there is another difference.
"Φίλαθλος" is someone who likes, who loves, who's a "friend" (φίλαθλος = φίλος + άθλος) of a sport (or the team, but mostly the sport itself).
"Οπαδός" is someone who likes or loves the actual team, not necessarily the sport. Team comes before the sport for them.
I hope I helped you enough^.^