PTSD is a diagnosis where some of the symptoms are intrusion thoughts—the thoughts are coming back, and you have avoidance. With an operational stress injury, it's not only a trauma that can cause a condition to develop.
As Bryson was saying earlier, the most common mental health problem in the Canadian Forces is depression. Anxiety and depression are specific diagnoses that may be related to just the stress of being in operation. If you're there and you're away from your family, etc., and you're always nervous and you develop depression, we call those operational stress injuries. PTSD is a trauma that has its own set of criteria and symptoms associated with it.