From what I hear, not too many guilds still farm OMM on a regular basis, that should give you the idea of how difficult the encounter is. Not so much difficult I guess, but as someone else said, there is a huge luck factor involved that can be discouraging, and makes guilds avoid doing him, in favor of other, more "reliable" mobs to kill. No idea on Mayong though, we have to kill Roley and Performer first, but all the rest of the Demi Plane mobs are much much easier then OMM in my opinion.
So, congrats to you, I know if felt great when we finally did it (think it took us about 10 tries I think to finally kill him - so it wasn't all that bad for us compared with others - maybe that time we got lucky

).