So far I’ve found using a Long neck and tranq darts to get it to the point where it starts to stumble and a blue ring appears over its head. Once it does that you hop on and ride it smashing these ice crystals that it shoots out. You also have to roll smaller creatures inside the giant ball of poo it rolls. The higher the level the Ossidon the more Ice crystals you have to smash and more creatures you have to roll up. I found that a level 35 I had to smash 18 crystals and roll up 8 creatures. Once you do that it pops you off and you have to repeat the process. You only have 1:50 to ride it around regardless of level