First of all, never say never in medicine. If you look in the psychiatric literature, I doubt you will find the term “OSI” anywhere. It's a term that was coined by the Canadian Forces to broaden the discussion of mental health issues beyond PTSD. It's any persistent mental health disorder that can be linked to your service with the CF.
While PTSD usually has a pretty abrupt cause, that doesn't mean that from that moment on you are suffering from the symptoms. Sometimes it takes a while for the symptoms to rise to a bothersome point.