Gee, I turn my back for a year and look what happens.
Just ran across an unusual discussion on Harry Potter Wiki. They found what they believe is the REAL ACTUAL ODO THE HERO SONG, COMPLETE, ONLINE!!
How do they know it's the real one?
Because the last four lines are exactly what JKR had in her book! Wow! It must be real!
Um...
First off, the song in question is very familiar. It begins --
Now gather 'round, fellows, let's sing us the saga
Of Odo the hero, born one happy day -
His father from Giggleswick, in all its glory;
His mother from Wigglesworth, farther away.
Um... yeah, it's mine, from Fanfiction.net. As the disclaim statement suggests, only the last lines are JKR's; the rest are pure fanfilk... just like the other dozen or so Odo poems by other folk in FF.n's Odo The Hero Community.
There's also a painful discussion about whether it's fanon or not.
Read it and groan:
http://harrypotter.wikia.com/wiki/Talk:Odo_the_Hero .