They make the decision based upon the facts of the case. They're not making a diagnosis with respect to the condition. The diagnosis of osteoarthritis is plainly made. They are required to evaluate the evidence put before them, just as a judge would in the normal course of circumstances in which credible medical evidence is led saying that this condition may or may not be related entirely to service.
They're also guided by the entitlement eligibility guidelines, which set out the requirements for determining whether osteoarthritis might be related to accumulated joint trauma. They factually say that if an individual did the following things for the following number of hours a day over a particular period of time, then there is a relationship to service.