The XO Ox is in the market! I wanted him so bad a couple years ago. He says not available by breeding, so I don't know if he is able to be bred right now, but the combo used to be Love Bug and Kissy Cat. I don't know if using that combo would work, or just using earth and plant might work, or if he isn't breedable at all. I just wanted to share my knowledge in case it helps anyone out.
Update: Well, darn it, it won't let me breed my Kissy Cat or my Love Bug, so I guess that won't work. That kind of makes me think that the OX Ox is, therefore, unbreedable since the only way to breed him before was with those 2 animals.