Ok - gonna be a long one. First, pay attention to level as you would with any other tame to get the one you want. Try to isolate your ossidon - this will be difficult because he attacks nearly everything. You will have to tranq it. Asc level rifle and regular tranq darts will work but take more. Once he is sufficiently tranqed, blue stars will revolve around his head. You only have a few moments to then jump on his back and begin rolling around a bolus. You have to roll the bolus over the icicles he shoots AND try to pick up smaller creatures in the bolus. Each of these adds to the overall taming. Continue to roll around until the timer expires and you'll be thrown off. Best option is then to kill any strong creatures around the ossidon so it doesn't die. Let him kill weaker creatures to get his life back and repeat tranqing. Continue this process until tamed.